Continuous process for producing electrodes for supercapacitors having high energy densities

ABSTRACT

A process for producing a supercapacitor cell, comprising: (a) Continuously feeding a conductive porous layer to a cathode material impregnation zone, wherein the conductive porous layer contains interconnected electron-conducting pathways and at least 70% by volume of pores; (b) Impregnating a wet cathode active material mixture (containing a cathode active material and an optional conductive additive mixed with a liquid electrolyte) into pores of this porous layer to form a cathode electrode; (c) Preparing an anode electrode in a similar manner; and (d) Stacking an anode electrode, a porous separator, and a cathode electrode to form the supercapacitor, wherein the anode electrode and/or the cathode electrode has a thickness no less than 100 μm; and/or wherein the anode or cathode active material constitutes an electrode active material loading no less than 7 mg/cm 2  in the anode or the cathode.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 14/756,777, filed on Oct. 13, 2015, which is herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to the field of supercapacitorsor ultracapacitors, and more particularly to the production processesfor supercapacitor electrodes and cells.

BACKGROUND OF THE INVENTION

Electrochemical capacitors (ECs), also known as ultracapacitors orsupercapacitors, are being considered for uses in hybrid electricvehicles (EVs) where they can supplement a battery used in an electriccar to provide bursts of power needed for rapid acceleration, thebiggest technical hurdle to making battery-powered cars commerciallyviable. A battery would still be used for cruising, but supercapacitors(with their ability to release energy much more quickly than batteries)would kick in whenever the car needs to accelerate for merging, passing,emergency maneuvers, and hill climbing. The EC must also storesufficient energy to provide an acceptable driving range. To be cost-,volume-, and weight-effective compared to additional battery capacitythey must combine adequate energy densities (volumetric and gravimetric)and power densities with long cycle life, and meet cost targets as well.

ECs are also gaining acceptance in the electronics industry as systemdesigners become familiar with their attributes and benefits. ECs wereoriginally developed to provide large bursts of driving energy fororbital lasers. In complementary metal oxide semiconductor (CMOS) memorybackup applications, for instance, a one-Farad EC having a volume ofonly one-half cubic inch can replace nickel-cadmium or lithium batteriesand provide backup power for months. For a given applied voltage, thestored energy in an EC associated with a given charge is half thatstorable in a corresponding battery system for passage of the samecharge. Nevertheless, ECs are extremely attractive power sources.Compared with batteries, they require no maintenance, offer much highercycle-life, require a very simple charging circuit, experience no“memory effect,” and are generally much safer. Physical rather thanchemical energy storage is the key reason for their safe operation andextraordinarily high cycle-life. Perhaps most importantly, capacitorsoffer higher power density than batteries.

The high volumetric capacitance density of an EC relative toconventional capacitors (10 to 100 times greater than conventionalcapacitors) derives from using porous electrodes to create a largeeffective “plate area” and from storing energy in the diffuse doublelayer. This double layer, created naturally at a solid-electrolyteinterface when voltage is imposed, has a thickness of only about 1 nm,thus forming an extremely small effective “plate separation.” Such asupercapacitor is commonly referred to as an electric double layercapacitor (EDLC). The double layer capacitor is based on a high surfacearea electrode material, such as activated carbon, immersed in a liquidelectrolyte. A polarized double layer is formed at electrode-electrolyteinterfaces providing high capacitance. This implies that the specificcapacitance of a supercapacitor is directly proportional to the specificsurface area of the electrode material. This surface area must beaccessible by electrolyte and the resulting interfacial zones must besufficiently large to accommodate the so-called electric double-layercharges.

In some ECs, stored energy is further augmented by pseudo-capacitanceeffects, occurring again at the solid-electrolyte interface due toelectrochemical phenomena such as the redox charge transfer.

However, there are several serious technical issues associated withcurrent state-of-the-art ECs or supercapacitors:

-   (1) Experience with ECs based on activated carbon electrodes shows    that the experimentally measured capacitance is always much lower    than the geometrical capacitance calculated from the measured    surface area and the width of the dipole layer. For very high    surface area carbons, typically only about 10-20 percent of the    “theoretical” capacitance was observed. This disappointing    performance is related to the presence of micro-pores (<2 nm, mostly    <1 nm) and ascribed to inaccessibility of some pores by the    electrolyte, wetting deficiencies, and/or the inability of a double    layer to form successfully in pores in which the oppositely charged    surfaces are less than about 1-2 nm apart. In activated carbons,    depending on the source of the carbon and the heat treatment    temperature, a surprising amount of surfaces can be in the form of    such micro-pores.-   (2) Despite the high gravimetric capacitances at the electrode level    (based on active material weights alone) as frequently claimed in    open literature and patent documents, these electrodes unfortunately    fail to provide energy storage devices with high capacities at the    supercapacitor cell or pack level (based on the total cell weight or    pack weight). This is due to the notion that, in these reports, the    actual mass loadings of the electrodes and the apparent densities    for the active materials are too low. In most cases, the active    material mass loadings of the electrodes (areal density) is    significantly lower than 10 mg/cm² (areal density=the amount of    active materials per electrode cross-sectional area along the    electrode thickness direction) and the apparent volume density or    tap density of the active material is typically less than 0.75    g/cm⁻³ (more typically less than 0.5 g/cm⁻³ and most typically less    than 0.3 g/cm⁻³) even for relatively large particles of activated    carbon.

The low mass loading is primarily due to the inability to obtain thickerelectrodes (thicker than 100-200 μm) using the conventional slurrycoating procedure. This is not a trivial task as one might think, and inreality the electrode thickness is not a design parameter that can bearbitrarily and freely varied for the purpose of optimizing the cellperformance. Contrarily, thicker samples tend to become extremelybrittle or of poor structural integrity and would also require the useof large amounts of binder resin. These problems are particularly acutefor graphene material-based electrodes. It has not been previouslypossible to produce graphene-based electrodes that are thicker than 150μm and remain highly porous with pores remaining fully accessible toliquid electrolyte. The low areal densities and low volume densities(related to thin electrodes and poor packing density) result inrelatively low volumetric capacitances and low volumetric energy densityof the supercapacitor cells.

With the growing demand for more compact and portable energy storagesystems, there is keen interest to increase the utilization of thevolume of the energy storage devices. Novel electrode materials anddesigns that enable high volumetric capacitances and high mass loadingsare essential to achieving improved cell volumetric capacitances andenergy densities.

-   (3) During the past decade, much work has been conducted to develop    electrode materials with increased volumetric capacitances utilizing    porous carbon-based materials, such as graphene, carbon    nanotube-based composites, porous graphite oxide, and porous    mesocarbon. Although these experimental supercapacitors featuring    such electrode materials can be charged and discharged at high rates    and also exhibit large volumetric electrode capacitances (100 to 200    F/cm³ in most cases, based on the electrode volume), their typical    active mass loading of <1 mg/cm², tap density of <0.2 g/cm⁻³, and    electrode thicknesses of up to tens of micrometers (<<100 μm) are    still significantly lower than those used in most commercially    available electrochemical capacitors (i.e. 10 mg/cm², 100-200 μm),    which results in energy storage devices with relatively low areal    and volumetric capacitances and low volumetric energy densities. A    typical graphene sheet is shown in FIG. 2.-   (4) For graphene-based supercapacitors, there are additional    problems that remain to be solved, explained below:

Nano graphene materials have recently been found to exhibitexceptionally high thermal conductivity, high electrical conductivity,and high strength. Another outstanding characteristic of graphene is itsexceptionally high specific surface area. A single graphene sheetprovides a specific external surface area of approximately 2,675 m²/g(that is accessible by liquid electrolyte), as opposed to the exteriorsurface area of approximately 1,300 m²/g provided by a correspondingsingle-wall CNT (interior surface not accessible by electrolyte). Theelectrical conductivity of graphene is slightly higher than that ofCNTs.

The instant applicants (A. Zhamu and B. Z. Jang) and their colleagueswere the first to investigate graphene- and other nano graphite-basednanomaterials for supercapacitor application [Please see Refs. 1-5below; the 1^(st) patent application was submitted in 2006 and issued in2009]. After 2008, researchers began to realize the significance of nanographene materials for supercapacitor applications.

LIST OF REFERENCES

-   1. Lulu Song, A. Zhamu, Jiusheng Guo, and B. Z. Jang “Nano-scaled    Graphene Plate Nanocomposites for Supercapacitor Electrodes” U.S.    Pat. No. 7,623,340 (Nov. 24, 2009).-   2. Aruna Zhamu and Bor Z. Jang, “Process for Producing Nano-scaled    Graphene Platelet Nanocomposite Electrodes for Supercapacitors,”    U.S. patent application Ser. No. 11/906,786 (Oct. 7, 2004) (U.S.    Pat. Pub. No. 2009-0092747).-   3. Aruna Zhamu and Bor Z. Jang, “Graphite-Carbon Composite    Electrodes for Supercapacitors” U.S. patent application Ser. No.    11/895,657 (Aug. 27, 2007) (U.S. Pat. Pub. No. 2009-0059474).-   4. Aruna Zhamu and Bor Z. Jang, “Method of Producing Graphite-Carbon    Composite Electrodes for Supercapacitors” U.S. patent application    Ser. No. 11/895,588 (Aug. 27, 2007) (U.S. Pat. Pub. No.    2009-0061312).-   5. Aruna Zhamu and Bor Z. Jang, “Graphene Nanocomposites for    Electrochemical cell Electrodes,” U.S. patent application Ser. No.    12/220,651 (Jul. 28, 2008) (U.S. Pat. Pub. No. 2010-0021819).

However, individual nano graphene sheets have a great tendency tore-stack themselves, effectively reducing the specific surface areasthat are accessible by the electrolyte in a supercapacitor electrode.The significance of this graphene sheet overlap issue may be illustratedas follows: For a nano graphene platelet with dimensions of l (length)×w(width)×t (thickness) and density ρ, the estimated surface area per unitmass is S/m=(2/φ (1/l+1/w+1/t). With ρ≅2.2 g/cm³, l=100 nm, w=100 nm,and t=0.34 nm (single layer), we have an impressive S/m value of 2,675m²/g, which is much greater than that of most commercially availablecarbon black or activated carbon materials used in the state-of-the-artsupercapacitor. If two single-layer graphene sheets stack to form adouble-layer graphene, the specific surface area is reduced to 1,345m²/g. For a three-layer graphene, t=1 nm, we have S/m=906 m²/g. If morelayers are stacked together, the specific surface area would be furthersignificantly reduced.

These calculations suggest that it is critically important to find a wayto prevent individual graphene sheets to re-stack and, even if theypartially re-stack, the resulting multi-layer structure would still haveinter-layer pores of adequate sizes. These pores must be sufficientlylarge to allow for accessibility by the electrolyte and to enable theformation of electric double-layer charges, which typically require apore size of at least 1 nm, more preferably at least 2 nm. However,these pores or inter-graphene spacings must also be sufficiently smallto ensure a large tap density (electrode mass per unit volume).Unfortunately, the typical tap density of graphene-based electrode isless than 0.3 g/cm³, and most typically <<0.1 g/cm³. To a great extent,the requirement to have large pore sizes and high porosity level and therequirement to have a high tap density are considered mutually exclusivein supercapacitors.

Another major technical barrier to using graphene sheets as asupercapacitor electrode active material is the challenge of depositinga thick active material layer onto the surface of a solid currentcollector (e.g. Al foil) using the conventional graphene-solvent slurrycoating procedure. In such an electrode, the graphene electrodetypically requires a large amount of a binder resin (hence,significantly reduced active material proportion vs. non-active oroverhead materials/components). In addition, any electrode prepared inthis manner that is thicker than 50 μm is brittle and weak, having agreat tendency to delaminate and micro-crack. These characteristics havemade the supercapacitor electrode thickness not a design parameter, buta manufacturing-limited feature. A supercapacitor designer cannot freelyincrease the electrode thickness. There has been no effective solutionto these problems.

Therefore, there is a clear and urgent need for supercapacitors thathave a high active material mass loading (high areal density), activematerials with a high apparent density (high tap density), highelectrode thickness with structural integrity and without significantlydecreased electron and ion transport rates (e.g. without large electrontransport resistance), high volumetric capacitance, and high volumetricenergy density. For graphene-based electrodes, one must also overcomeproblems such as re-stacking of graphene sheets, the demand for largeproportion of a binder resin, and difficulty in producing thick grapheneelectrode layers.

SUMMARY OF THE INVENTION

The present invention provides a process for producing electrodes of asupercapacitor cell and further for producing the supercapacitor cellhaving a high active material mass loading, exceptionally low overhead(ancillary component) weight and volume (relative to the active materialmass and volume), high volumetric capacitance, and unprecedentedly highvolumetric energy density.

In one embodiment, the invented process comprises: (A) continuouslyfeeding a first electrically conductive porous layer to a cathodematerial impregnation zone, wherein the first conductive porous layerhas two opposed porous surfaces and contains interconnectedelectron-conducting pathways and, preferably, at least 70% by volume ofpores; (B) impregnating a wet cathode active material mixture into thefirst electrically conductive porous layer from at least one of the twoporous surfaces to form a cathode electrode, wherein the wet cathodeactive material mixture contains a cathode active material and anoptional conductive additive mixed with a first liquid electrolyte; (C)continuously feeding a second electrically conductive porous layer to ananode material impregnation zone, wherein the second conductive porouslayer has two opposed porous surfaces and contains interconnectedelectron-conducting pathways and, preferably, at least 70% by volume ofpores; (D) impregnating a wet anode active material mixture into thesecond electrically conductive porous layer from at least one of the twoporous surfaces to form an anode electrode, wherein the wet anode activematerial mixture contains an anode active material and an optionalconductive additive mixed with a second liquid electrolyte; and (E)stacking an anode electrode, a porous separator, and a cathode electrodeto form an alkali metal battery, wherein the anode electrode and/or thecathode electrode has a thickness no less than 100 μm and/or wherein theanode active material or cathode active material constitutes anelectrode active material loading no less than 7 mg/cm² in the anode orcathode electrode.

The steps (A) and (B) combined, or (C) and (D) combined, constitute aprocess for producing the respective electrode (cathode or anode). Thus,the present invention also provides a process for producing an electrodefor a supercapacitor cell, the process comprising: (A) continuouslyfeeding an electrically conductive porous layer to an anode or cathodematerial impregnation zone, wherein the conductive porous layer has twoopposed porous surfaces and contains interconnected electron-conductingpathways and, preferably, at least 70% by volume of pores; and (B)impregnating a wet anode or cathode active material mixture into theelectrically conductive porous layer from at least one of the two poroussurfaces to form an anode electrode or cathode electrode, wherein thewet anode or cathode active material mixture contains an anode orcathode active material and an optional conductive additive mixed with aliquid electrolyte.

In the electrode-producing process or the cell-producing process, step(A) and step (B) include delivering, continuously or intermittently ondemand, the wet cathode active material mixture to the at least oneporous surface through spraying, printing, coating, casting, conveyorfilm delivery, and/or roller surface delivery. The step (C) and step (D)may also include delivering, continuously or intermittently on demand,the wet anode active material mixture to at least one porous surfacethrough spraying, printing, coating, casting, conveyor film delivery,and/or roller surface delivery.

The first conductive porous layer and the second conductive porous layercan be made of the same material or different materials. Each layercontains interconnected 2D or 3D network of electron-conducting paths asa cathode current collector or anode current collectors having pores toaccommodate liquid electrolyte and a cathode active material or an anodeactive material. The first and/or second conductive porous layer has athickness no less than 100 μm (preferably greater than 200 μm, morepreferably greater than 300 μm, further preferably greater than 400 μm,still more preferably greater than 500 μm, and most preferably up to5,000 μm or 5 mm (actually, there is no limitation on the porous layerthickness or final electrode thickness). The conductive porous layerpreferably has at least 70% by volume of pores (more preferably at least80% porosity, still more preferably at least 90%, and most preferably atleast 95%).

In some embodiments, the anode active material and/or the cathode activematerial contains multiple particles of a carbon material and/ormultiple graphene sheets, wherein the multiple graphene sheets containsingle-layer graphene or few-layer graphene (each having from 1 to 10graphene planes) and the multiple particles of carbon material have aspecific surface area no less than 500 m²/g (preferably >1,000 m²/g,more preferably >1,500 m²/g, further more preferably >2,000 m²/g, stillmore preferably >2,500 m²/g, and most preferably more preferably >3,000m²/g) when measured in a dried state (the practical limit is 3,500 m²/gfor carbon-based materials). In the final electrode, the anode activematerial or the cathode active material constitutes an electrode activematerial loading no less than 7 mg/cm² (preferably no less than 10mg/cm², more preferably no less than 15 mg/cm², further more preferablyno less than 20 mg/cm², still more preferably no less than 25 mg/cm²,and most preferably no less than 30 mg/cm²) in the anode or the cathode.

The electrically conductive porous layer herein refers to a structurethat contains a high pore volume (>70% or more) and an interconnectednetwork of electron-conducting paths. This can be, for instance,end-connected 2D mats, webs, chicken wire-like metal screens, etc. Asillustrated in FIG. 3(A), FIG. 3(B), FIG. 3(C) and FIG. 3(D), this canalso be metal foam, conductive polymer foam, graphite foam, carbon foam,or graphene foam, etc., wherein pore walls contain conductive materials.The pore volume (e.g. >70%) of a conductive porous layer (serving as acurrent collector) is a critically important requirement to ensure alarge proportion of active materials being accommodated in the currentcollector. Based on this criterion, conventional paper or textiles madeof natural and/or synthetic fibers do not meet this requirement sincethey do not have a sufficient amount of properly sized pores.

The pore sizes in the first and/or second conductive porous layers arepreferably in the range from 10 nm to 50 μm, more preferably from 100 nmto 20 μm, further preferably from 500 nm to 10 μm, and most preferablyfrom 1 μm to 5 μm. These pore size ranges are designed to accommodategraphene sheets, which are typically from 10 nm to 50 μm in length/widthand most typically from 100 nm to 20 μm, further typically from 200 nmto 10 μm, and most typically from 0.5 μm to 5 μm. More significantly,however, since all active material particles (e.g. graphene sheets,carbon particles, etc.) are, on average, within a distance of 25 μm froma pore wall in the 3D foam structure, electrons (charges) can be readilycollected from the electric double layers near the activematerial-electrolyte interface. This is in contrast to the notion thatsome electrons in the conventional thick electrode of prior artsupercapacitors (e.g. wherein an activated carbon layer >100 μm inthickness is coated onto a surface of a solid Al foil current collector12 μm thick) must travel at least 100 μm to get collected by a currentcollector (meaning a larger internal resistance and reduced ability todeliver a higher power).

In general, the wet anode active material mixture and the wet cathodeactive material mixture are identical in composition in a symmetricsupercapacitor, but they can be different in composition. The liquidelectrolytes can be an aqueous liquid, organic liquid, ionic liquid(ionic salt having a melting temperature lower than 100° C., preferablylower than room temperature, 25° C.), or mixture of an ionic liquid andan organic liquid at a ratio from 1/100 to 100/1. The organic liquid isdesirable, but the ionic liquid is preferred.

A supercapacitor active material is a material responsible for storingcharges via the nearby electric double layers or via the redox mechanismin the supercapacitor cell. As an active material, the carbon materialmay be selected from activated carbon, activated mesocarbon micro beads(activated MCMBs), activated graphite, activated or chemically etchedcarbon black, activated hard carbon, activated soft carbon, carbonnanotube, carbon nanofiber, activated carbon fiber, activated graphitefiber, exfoliated graphite worms, activated graphite worms, activatedexpanded graphite flakes, or a combination thereof.

In a preferred embodiment, the first and/or second conductive porouslayer has a thickness no less than 200 μm, has at least 85% by volume ofpores (prior to impregnation of the wet electrode active materialmixture), and/or the electrode active material loading is no less than10 mg/cm². Preferably, the active material in the two electrodes (anodeand cathode) combined occupies at least 30% by weight or by volume ofthe entire supercapacitor cell. This weight or volume proportion of theactive material is preferably no less than 40%, further preferably noless than 50%, and mote preferably no less than 60%. These have not beenpossible with conventional supercapacitors.

In a further preferred embodiment, the first and/or second conductiveporous layer structure has a thickness no less than 300 μm, has at least90% by volume of pores, and/or the electrode active material loading isno less than 15 mg/cm².

In a more preferred embodiment, the first and/or second conductiveporous layer structure has a thickness no less than 400 μm, has at least95% by volume of pores, and/or the electrode active material loading isno less than 20 mg/cm². The conductive porous layer thickness is mostpreferably from 400 μm to 5 mm.

In certain embodiments, the first and/or second conductive porous layerstructure is selected from metal foam, metal web or screen, perforatedmetal sheet-based 3-D structure, metal fiber mat, metal nanowire mat,conductive polymer nanofiber mat, conductive polymer foam, conductivepolymer-coated fiber foam, carbon foam, graphite foam, carbon aerogel,carbon xerogel, graphene foam, graphene oxide foam, reduced grapheneoxide foam, carbon fiber foam, graphite fiber foam, exfoliated graphitefoam, or a combination thereof.

As a supercapacitor active material, graphene sheets may be selectedfrom the group consisting of pristine graphene, graphene oxide, reducedgraphene oxide, graphene fluoride, graphene chloride, graphene bromide,graphene iodide, hydrogenated graphene, nitrogenated graphene,boron-doped graphene, nitrogen-doped graphene, chemically functionalizedgraphene, physically or chemically activated or etched versions thereof,and combinations thereof. In an embodiment, the anode or the cathodecontains graphene sheets as the only electrode active material and doesnot contain any other electrode active material.

In certain embodiments, the anode or the cathode contains the followingmaterials as the only electrode active material in the anode or cathode:(a) graphene sheets alone; (b) graphene sheets mixed with a carbonmaterial; (c) graphene sheets mixed with a partner material that forms aredox pair with the graphene sheets to develop pseudo-capacitance; or(d) graphene sheets and a carbon material mixed with a partner material(e.g. a conducting polymer or metal oxide) that forms a redox pair withgraphene sheets or the carbon material to develop pseudo-capacitance,and wherein there is no other electrode active material in the anode orcathode.

The anode active material and the cathode active material can be thesame material or different materials. Preferably, the volume ratio ofthe anode active material-to-liquid electrolyte in the first dispersionis from 1/5 to 20/1 (preferably from 1/3 to 5/1) and/or the volume ratioof cathode active material-to-the liquid electrolyte in the seconddispersion is from 1/5 to 20/1 (preferably from 1/3 to 5/1).

In certain embodiments, the anode active material or cathode activematerial contains a carbon and/or graphene material and further containsa redox pair partner material selected from a metal oxide, a conductingpolymer, an organic material, a non-graphene carbon material, aninorganic material, or a combination thereof, wherein the partnermaterial, in combination with graphene or a carbon material, form aredox pair for developing pseudo-capacitance. The metal oxide may beselected from RuO₂, IrO₂, NiO, MnO₂, VO₂, V₂O₅, V₃O₈, TiO₂, Cr₂O₃,Co₂O₃, Co₃O₄, PbO₂, Ag₂O, or a combination thereof. The inorganicmaterial may be selected from a metal carbide, metal nitride, metalboride, metal dichalcogenide, or a combination thereof. Conductingpolymers preferably contain an intrinsically conductive polymer, such aspolyacetylene, polypyrrole, polyaniline, polythiophene, or theirderivatives.

In some embodiments, the metal oxide or inorganic material may beselected from an oxide, dichalcogenide, trichalcogenide, sulfide,selenide, or telluride of niobium, zirconium, molybdenum, hafnium,tantalum, tungsten, titanium, vanadium, chromium, cobalt, manganese,iron, or nickel in a nanowire, nanodisc, nanoribbon, or nanoplateletform. Preferably, the inorganic material is selected from nanodiscs,nanoplatelets, nano-coating, or nanosheets of an inorganic materialselected from: (a) bismuth selenide or bismuth telluride, (b) transitionmetal dichalcogenide or trichalcogenide, (c) sulfide, selenide, ortelluride of niobium, zirconium, molybdenum, hafnium, tantalum,tungsten, titanium, cobalt, manganese, iron, nickel, or a transitionmetal; (d) boron nitride, or (e) a combination thereof; wherein thediscs, platelets, or sheets have a thickness less than 100 nm.

In an embodiment, we have a supercapacitor-type cathode (an electricdouble layer or redox pair type electrode), but the anode is abattery-like anode that intercalates/deintercalates lithium or sodiumions (e.g. containing prelithiated graphite particles, pre-sodiatedcarbon particles, or prelithiated Si particles) and the resultingsupercapacitor is a lithium ion capacitor or sodium-ion capacitor (stilla capacitor, not a battery). Thus, the invention also provides a processfor producing a special class of supercapacitor cell (i.e. a lithium ioncapacitor, LIC, or sodium-ion capacitor, NIC). The process includes: (A)Continuously feeding a first electrically conductive porous layer to acathode material impregnation zone, wherein the first conductive porouslayer has two opposed porous surfaces and contains interconnectedelectron-conducting pathways and, preferably, at least 70% by volume ofpores; (B) Impregnating a wet cathode active material mixture into thefirst electrically conductive porous layer from at least one of the twoporous surfaces to form a cathode electrode, wherein the wet cathodeactive material mixture contains a cathode active material and anoptional conductive additive mixed with a first liquid electrolyte; (C)Continuously supplying an anode electrode, which contains a prelithiatedor pre-sodiated anode active material; and (D) Stacking an anodeelectrode, a porous separator, and a cathode electrode to form alithium-ion capacitor or sodium-ion capacitor, wherein the cathodeelectrode has a thickness no less than 100 μm (preferably >200 μm, morepreferably >300 μm, further more preferably >400 μm, still morepreferably >500 μm, and most preferably >600 μm) and the cathode activematerial constitutes an electrode active material loading no less than 7mg/cm² in said cathode electrode. Preferably, the cathode activematerial constitutes an electrode active material loading no less than10 mg/cm² (preferably >15 mg/cm², more preferably >20 mg/cm², still morepreferably >25 mg/cm², and most preferably >30 mg/cm²).

There are at least two ways to prepare the anode electrode layer; theconventional process and the presently invented process. Theconventional process includes mixing graphite particles in NMP solventto form a slurry, which is coated on one or two surfaces of an anodecurrent collector (e.g. non-porous, thin Cu foil). The solvent is thenremoved to obtain dried electrode and a liquid electrolyte is injectedinto the anode side after the battery cell is assembled and housed in apackaging envelop. More preferably, the anode electrode for thepresently invented LIC or NIC is also made by the presently inventedprocess described above. This inventive process includes (A)Continuously feeding a second electrically conductive porous layer to ananode material impregnation zone, wherein the second conductive porouslayer has two opposed porous surfaces and contain interconnectedelectron-conducting pathways and, preferably, at least 70% by volume ofpores; and (B) Impregnating a wet anode active material mixture into thesecond electrically conductive porous layer from at least one of the twoporous surfaces to form an electrode. The wet anode active materialmixture contains a liquid electrolyte and an anode active materialselected from prelithiated or pre-sodiated versions of graphiteparticles, carbon particles, Si nanoparticles, Sn nanoparticles, or anyother commonly used anode active materials for lithium-ion batteries orsodium-ion batteries.

In a lithium-ion capacitor (LIC), the anode active material may beselected from the group consisting of: (a) prelithiated particles ofnatural graphite, artificial graphite, mesocarbon microbeads (MCMB), andcarbon; (b) prelithiated particles or coating of Silicon (Si), germanium(Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn),aluminum (Al), nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti),iron (Fe), and cadmium (Cd); (c) prelithiated alloys or intermetalliccompounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements,wherein said alloys or compounds are stoichiometric ornon-stoichiometric; (d) prelithiated oxides, carbides, nitrides,sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites;and (e) prelithiated graphene sheets; and combinations thereof.

In a sodium-ion capacitor, the anode active material contains apre-sodiated version of petroleum coke, carbon black, amorphous carbon,activated carbon, hard carbon, soft carbon, templated carbon, hollowcarbon nanowires, hollow carbon sphere, or titanate, or a sodiumintercalation compound selected from NaTi₂(PO₄)₃, Na₂Ti₃O₇, Na₂C₈H₄O₄,Na₂TP, Na_(x)TiO₂ (x=0.2 to 1.0), Na₂C₈H₄O₄, carboxylate based material,C₈H₄Na₂O₄, C₈H₆O₄, C₈H₅NaO₄, C₈Na₂F₄O₄, C₁₀H₂Na₄O₈, C₁₄H₄O₆, C₁₄H₄Na₄O₈,or a combination thereof.

In a sodium-ion capacitor, the anode active material contains a sodiumintercalation compound selected from the following groups of materials:(a) Sodium-doped silicon (Si), germanium (Ge), tin (Sn), lead (Pb),antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti),cobalt (Co), nickel (Ni), manganese (Mn), cadmium (Cd), and mixturesthereof, (b) Sodium-containing alloys or intermetallic compounds of Si,Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c)Sodium-containing oxides, carbides, nitrides, sulfides, phosphides,selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof; (d) Sodiumsalts; and (e) Graphene sheets pre-loaded with sodium or potassium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) Schematic of a prior art supercapacitor cell.

FIG. 1(B) Schematic of a presently invented supercapacitor cell,comprising a conductive porous layer (e.g. an anode current collector inthe form of a highly porous foam) impregnated with a wet anode activematerial mixture (e.g. liquid electrolyte and an anode active materialand optional conductive additive), a porous separator, and a cathodecurrent collector in the form of a highly porous foam impregnated with awet cathode active material mixture.

FIG. 1(C) Four examples that schematically illustrate the presentlyinvented process for producing an electrode (anode or cathode) of asupercapacitor cell.

FIG. 1(D) Another example to schematically illustrate the presentlyinvented process to produce an electrode (anode or cathode).

FIG. 1(E) Schematic of a presently invented process for continuouslyproducing a supercapacitor cell by combining and laminating an anodeelectrode, separator, and cathode electrode (illustrated in SchematicF), and that for continuously producing a an asymmetric supercapacitor(lithium-ion capacitor or sodium-ion capacitor) laminate in an automatedmanner (Schematic G); the latter supercapacitor comprising an anodeelectrode (e.g. a porous conductive layer impregnated with graphiteparticles and liquid electrolyte), a porous separator, and a cathodeelectrode prepared by a presently invented process.

FIG. 2 An electron microscopic image of typical graphene sheets.

FIG. 3(A) Examples of conductive porous layers: metal grid/mesh andcarbon nanofiber mat.

FIG. 3(B) Examples of conductive porous layers: graphene foam and carbonfoam.

FIG. 3(C) Examples of conductive porous layers: graphite foam and Nifoam.

FIG. 3(D) Examples of conductive porous layers: Cu foam and stainlesssteel foam.

FIG. 4(A) Schematic of a commonly used process for producing exfoliatedgraphite, expanded graphite flakes (thickness >100 nm), and graphenesheets (thickness <100 nm, more typically <10 nm, and can be as thin as0.34 nm).

FIG. 4(B) Schematic drawing to illustrate the processes for producingexfoliated graphite, expanded graphite flakes, and graphene sheets.

FIG. 5 Ragone plots (gravimetric and volumetric power density vs. energydensity) of symmetric supercapacitor (EDLC) cells containing reducedgraphene oxide (RGO) sheets as the electrode active material and EMIMBF4ionic liquid electrolyte. Supercapacitors were prepared according to anembodiment of instant invention and, for comparison, by the conventionalslurry coating of electrodes.

FIG. 6 Ragone plots (gravimetric and volumetric power density vs. energydensity) of symmetric supercapacitor (EDLC) cells containing activatedcarbon (AC) particles as the electrode active material and organicliquid electrolyte. Supercapacitors were prepared according to anembodiment of instant invention and by the conventional slurry coatingof electrodes.

FIG. 7(A) Ragone plots (gravimetric and volumetric power density vs.energy density) of lithium ion capacitor (LIC) cells containing pristinegraphene sheets as the electrode active material and lithium salt-PC/DECorganic liquid electrolyte. Supercapacitors were prepared according toan embodiment of instant invention and by the conventional slurrycoating of electrodes.

FIG. 7(B) Ragone plots (gravimetric and volumetric power density vs.energy density) of sodium ion capacitor (NIC) cells containing pristinegraphene sheets as the electrode active material and sodium salt-PC/DECorganic liquid electrolyte.

FIG. 8 The cell-level gravimetric and volumetric energy densitiesplotted over the achievable electrode thickness range of the AC-basedEDLC supercapacitors prepared via the conventional method and thepresently invented method. Legends: the gravimetric (♦) and volumetric(▴) energy density of the conventional supercapacitors and thegravimetric (▪) and volumetric (X) energy density of the inventivesupercapacitors. With the presently invented method, there is notheoretical limit on the electrode thickness that can be achieved.Typically, the practical electrode thickness is from 10 μm to 5,000 μm,more typically from 100 μm to 1,000 μm, and most typically from 200 μmto 800 μm.

FIG. 9 The cell-level gravimetric and volumetric energy densitiesplotted over the achievable electrode thickness range of the RGO-basedEDLC supercapacitors (organic liquid electrolyte) prepared via theconventional method and the presently invented method (easily achievedelectrode tap density of approximately 0.75 g/cm³).

FIG. 10 The cell-level gravimetric and volumetric energy densitiesplotted over the achievable electrode thickness range of the pristinegraphene-based EDLC supercapacitors (organic liquid electrolyte)prepared via the conventional method and the presently invented method(electrode tap density of approximately 1.15 g/cm³).

FIG. 11 The cell-level gravimetric and volumetric energy densitiesplotted over the achievable electrode thickness range of the pristinegraphene-based EDLC supercapacitors (ionic liquid electrolyte) preparedvia the conventional method and the presently invented method (electrodetap density of approximately 1.15 g/cm³).

FIG. 12 The cell-level gravimetric energy densities plotted over theachievable active material proportion (active material weight/total cellweight) for activated carbon-based EDLC supercapacitors (with organicliquid electrolyte).

FIG. 13 The cell-level gravimetric energy densities plotted over theachievable active material proportion (active material weight/total cellweight) in a supercapacitor cell for two series of pristinegraphene-based EDLC supercapacitors (all with organic liquidelectrolyte).

FIG. 14 The cell-level volumetric energy densities plotted over theachievable active material proportion (active material weight/total cellweight) for pristine graphene-based EDLC supercapacitors (with ionicliquid electrolyte).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be understood more readily by reference to thefollowing detailed description of the invention taken in connection withthe accompanying drawing figures, which form a part of this disclosure.It is to be understood that this invention is not limited to thespecific devices, methods, conditions or parameters described and/orshown herein, and that the terminology used herein is for the purpose ofdescribing particular embodiments by way of example only and is notintended to be limiting of the claimed invention.

As schematically illustrated in FIG. 1(A), a prior art supercapacitorcell is typically composed of an anode current collector 202 (e.g. Alfoil 12-15 μm thick), an anode active material layer 204 (containing ananode active material, such as activated carbon particles 232 andconductive additives that are bonded by a resin binder, such as PVDF)coated on the current collector, a porous separator 230, a cathodeactive material layer 208 (containing a cathode active material, such asactivated carbon particles 234, and conductive additives that are allbonded by a resin binder, not shown), a cathode current collector 206(e.g. Al foil), and a liquid electrolyte disposed in both the anodeactive material layer 204 (also simply referred to as the “anode layer”)and the cathode active material layer 208 (or simply “cathode layer”).The entire cell is encased in a protective housing, such as a thinplastic-aluminum foil laminate-based envelop. The prior artsupercapacitor cell is typically made by a process that includes thefollowing steps:

-   -   a) The first step is mixing particles of the anode active        material (e.g. activated carbon), a conductive filler (e.g.        graphite flakes), a resin binder (e.g. PVDF) in a solvent (e.g.        NMP) to form an anode slurry. On a separate basis, particles of        the cathode active material (e.g. activated carbon), a        conductive filler (e.g. acetylene black), a resin binder (e.g.        PVDF) are mixed and dispersed in a solvent (e.g. NMP) to form a        cathode slurry.    -   b) The second step includes coating the anode slurry onto one or        both primary surfaces of an anode current collector (e.g. Cu or        Al foil), drying the coated layer by vaporizing the solvent        (e.g. NMP) to form a dried anode electrode coated on Cu or Al        foil. Similarly, the cathode slurry is coated and dried to form        a dried cathode electrode coated on Al foil.    -   c) The third step includes laminating an anode/Al foil sheet, a        porous separator layer, and a cathode/Al foil sheet together to        form a 3-layer or 5-layer assembly, which is cut and slit into        desired sizes and stacked to form a rectangular structure (as an        example of shape) or rolled into a cylindrical cell structure.    -   d) The rectangular or cylindrical laminated structure is then        encased in an aluminum-plastic laminated envelope or steel        casing.    -   e) A liquid electrolyte is then injected into the laminated        structure to make a supercapacitor cell.

There are several serious problems associated with this conventionalprocess and the resulting supercapacitor cell:

-   -   1) It is very difficult to produce an activated carbon-based        electrode layer (anode layer or cathode layer) that is thicker        than 100 μm and practically impossible or impractical to produce        an electrode layer thicker than 200 μm. There are several        reasons why this is the case:        -   a. An electrode of 100 μm thickness typically requires a            heating zone of 30-50 meters long in a slurry coating            facility, which is too time consuming, too energy intensive,            and not cost-effective.        -   b. Thicker electrodes have a great tendency to get            delaminated and cracked.        -   c. For some electrode active materials, such as graphene            sheets, it has not been possible to produce an electrode            thicker than 50 μm in a real manufacturing environment on a            continuous basis. This is despite the notion that some            thicker electrodes have been claimed in open or patent            literature; these electrodes were prepared in a laboratory            on a small scale. In a laboratory setting, presumably one            could repeatedly add new materials to a layer and manually            consolidate the layer to increase the thickness of an            electrode. However, even with such a manual procedure (not            amenable to mass production), the resulting electrode            becomes very fragile and brittle.        -   d. This problem is even worse for graphene-based electrodes,            since repeated compressions lead to re-stacking of graphene            sheets and, hence, significantly reduced specific surface            area and reduced specific capacitance.    -   2) With a conventional process, as depicted in FIG. 1(A), the        actual mass loadings of the electrodes and the apparent        densities for the active materials are too low. In most cases,        the active material mass loadings of the electrodes (areal        density) is significantly lower than 10 mg/cm² and the apparent        volume density or tap density of the active material is        typically less than 0.75 g/cm³ (more typically less than 0.5        g/cm³ and most typically less than 0.3 g/cm³) even for        relatively large particles of activated carbon. In addition,        there are other non-active materials (e.g. conductive additive        and resin binder) that add additional weights and volumes to the        electrode without contributing to the cell capacity. These low        areal densities and low volume densities result in relatively        low volumetric capacitances and low volumetric energy density.    -   3) The conventional process requires dispersing electrode active        materials (anode active material and cathode active material) in        a liquid solvent (e.g. NMP) to make a slurry and, upon coating        on a current collector surface, the liquid solvent has to be        removed to dry the electrode layer. Once the anode and cathode        layers, along with a separator layer, are laminated together and        packaged in a housing to make a supercapacitor cell, one then        injects a liquid electrolyte into the cell. In actuality, one        makes the two electrodes wet, then makes the electrodes dry, and        finally makes them wet again. Such a wet-dry-wet process does        not sound like a good process at all.    -   4) NMP is not an environmentally friendly solvent; it is known        to potentially cause birth defects.    -   5) Current supercapacitors (e.g. symmetric supercapacitors or        electric double layer capacitors, EDLC) still suffer from a        relatively low gravimetric energy density and low volumetric        energy density. Commercially available EDLCs exhibit a        gravimetric energy density of approximately 6 Wh/kg and no        experimental EDLC cells have been reported to exhibit an energy        density higher than 10 Wh/kg (based on the total cell weight) at        room temperature. Although experimental supercapacitors exhibit        large volumetric electrode capacitances (100 to 200 F/cm³ in        most cases) at the electrode level (not the cell level), their        typical active mass loading of <1 mg/cm², tap density of <0.1        g/cm³, and electrode thicknesses of up to tens of micrometers        remain significantly lower than those used in most commercially        available electrochemical capacitors, resulting in energy        storage devices with relatively low areal and volumetric        capacities and low volumetric energy densities based on the cell        (device) weight.

In literature, the energy density data reported based on either theactive material weight alone or electrode weight cannot directlytranslate into the energy densities of a practical supercapacitor cellor device. The “overhead weight” or weights of other device components(binder, conductive additive, current collectors, separator,electrolyte, and packaging) must also be taken into account. Theconvention production process results in an active material proportionbeing less than 30% by weight of the total cell weight (<15% in somecases; e.g. for graphene-based active material).

The present invention provides a process for producing a supercapacitorcell having a high electrode thickness (thickness of the electrode thatcontains electrode active materials), high active material mass loading,low overhead weight and volume, high volumetric capacitance, and highvolumetric energy density. In addition, the manufacturing costs of thesupercapacitators produced by the presently invented process aresignificantly lower than those by conventional processes and are muchmore environmentally benign.

As illustrated in FIG. 1(B), the presently invented supercapacitor cellcomprises a conductive porous layer (e.g. an anode current collector inthe form of a highly porous foam) impregnated with a wet anode activematerial mixture (e.g. liquid electrolyte and an anode active materialand an optional conductive additive), a porous separator, and anotherconductive porous layer (e.g. a cathode current collector in the form ofa highly porous foam) impregnated with a wet cathode active materialmixture. The pores (e.g. 250) are filled with an anode active material(e.g. graphene sheets, 252) and liquid electrolyte (254). A binder resinis not desired.

In one embodiment of the present invention, as illustrated in FIG. 1(C)and FIG. 1(D), the invented process comprises continuously feeding anelectrically conductive porous layer (e.g. 304, 310, 322, or 330), froma feeder roller (not shown), into an active material impregnation zonewhere a wet active material mixture (e.g. slurry, suspension, orgel-like mass, such as 306 a, 306 b, 312 a, 312 b) of an electrodeactive material (e.g. activated carbon particles and/or graphene sheets)and an optional conductive additive is delivered to at least a poroussurface of the porous layer (e.g. 304 or 310 in Schematic A andSchematic B, respectively, of FIG. 1(C)). Using Schematic A as anexample, the wet active material mixture (306 a) is forced to impregnateinto the porous layer from both sides using one or two pairs of rollers(302 a, 302 b, 302 c, and 302 d) to form an impregnated active electrode308 (an anode or cathode). The conductive porous layer containsinterconnected electron-conducting pathways and at least 70% by volume(preferably >80%, more preferably >90%) of pores.

In Schematic B, two feeder rollers 316 a, 316 b are used to continuouslypay out two protective films 314 a, 314 b that support wet activematerial mixture layers 312 a, 312 b. These wet active material mixturelayers 312 a, 312 b can be delivered to the protective (supporting)films 314 a, 314 b using a broad array of procedures (e.g. printing,spraying, casting, coating, etc., which are well known in the art). Asthe conductive porous layer 310 moves though the gaps between two setsof rollers (318 a, 318 b, 318 c, 318 d), the wet active mixture materialis impregnated into the pores of the porous layer 310 to form an activematerial electrode 320 (an anode or cathode electrode layer) covered bytwo protective films 314 a, 314 b.

Using Schematic C as another example, two spraying devices 324 a, 324 bare used to dispense the wet active material mixture (325 a, 325 b) tothe two opposed porous surfaces of the conductive porous layer 322. Thewet active material mixture is forced to impregnate into the porouslayer from both sides using one or two pairs of rollers to form animpregnated active electrode 326 (an anode or cathode). Similarly, inSchematic D, two spraying devices 332 a, 332 b are used to dispense thewet active material mixture (333 a, 333 b) to the two opposed poroussurfaces of the conductive porous layer 330. The wet active materialmixture is forced to impregnate into the porous layer from both sidesusing one or two pairs of rollers to form an impregnated activeelectrode 338 (an anode or cathode).

The resulting electrode layer (anode or cathode electrode), afterconsolidation, has a thickness no less than 100 μm (preferably >200 m,further preferably >300 m, more preferably >400 μm; further morepreferably >500 μm, 600 μm, or even >1,000 μm; no theoretical limitationon this anode thickness). Consolidation may be accomplished with theapplication of a compressive stress (from rollers) to force the wetactive material mixture ingredients to infiltrate into the pores of theconductive porous layer. The conductive porous layer is also compressedtogether to form a current collector that essentially extends over thethickness of the entire electrode.

Another example, as illustrated in Schematic E of FIG. 1(D), theelectrode production process begins by continuously feeding a conductiveporous layer 356 from a feeder roller 340. The porous layer 356 isdirected by a roller 342 to get immersed into a wet active materialmixture mass 346 (slurry, suspension, gel, etc.) in a container 344. Theactive material mixture begins to impregnate into pores of the porouslayer 356 as it travels toward roller 342 b and emerges from thecontainer to feed into the gap between two rollers 348 a, 348 b. Twoprotective films 350 a, 350 b are concurrently fed from two respectiverollers 352 a, 352 b to cover the impregnated porous layer 354, whichmay be continuously collected on a rotating drum (a winding roller 355).The process is applicable to both the anode and the cathode electrodes.

As illustrated in Schematic F of FIG. 1(E), at least one anode electrode364 (e.g. produced by the presently invented process), a porousseparator 366, and at least one cathode electrode 368 (e.g. produced bythe presently invented process), may be unwound from rollers 360 a, 360b, and 360 c, respectively, laminated and consolidated together bymoving through a pair of rollers 362 a, 362 b to form a supercapacitorassembly 370. Such a supercapacitor assembly 370 can be slit and cutinto any desired shape and dimensions and sealed in a protectivehousing. It may be noted that a plurality of impregnated anode layerscan be stacked and compacted into one single anode electrode to achievea desired thickness. Similarly, a plurality of impregnated cathodelayers can be stacked and compacted into one single cathode electrode.

Alternatively, as illustrated in Schematic G of FIG. 1(E), an anodeelectrode 378 (e.g. a Cu foil coated with Li or Na intercalationcompound on two surfaces, or a conductive porous layer impregnated withparticles of Li or Na intercalation compound and liquid electrolyte), aporous separator 376, and a cathode electrode 374 (e.g. produced by thepresently invented process), may be unwound from rollers 370 c, 370 b,and 370 a, respectively, laminated and consolidated together by movingthrough a pair of rollers 362 a, 362 b to form a lithium-ion capacitor(LIC) or sodium-ion capacitor (NIC) assembly 380. Such a LIC or NICassembly 380 can be slit and cut into any desired shape and dimensionsand sealed in a protective housing.

The above are but several examples to illustrate how the presentlyinvented supercapacitor electrodes and supercapacitor cells can be madecontinuously, in an automated manner. These examples should not beconstrued as limiting the scope of the instant invention.

The first and/or second conductive porous layer has a thickness no lessthan 100 m (preferably greater than 200 μm, more preferably greater than300 μm, further preferably greater than 400 μm, and most preferablygreater than 500 μm; no theoretical limit on the electrode thickness)and preferably at least 80% by volume of pores (preferably at least 85%porosity, more preferably at least 90%, and most preferably at least95%). These porous layer structures have essentially a porosity level of80%-99% and remaining 1%-20% being pore walls (e.g. metal or graphiteskeleton). These pores are used to accommodate a mixture of activematerials (e.g. graphene sheets) and liquid electrolyte, where electricdouble layers of charges or redox pairs are present when thesupercapacitor is charged.

Preferably, substantially all of the pores in the porous layer arefilled with the electrode (anode or cathode) active material and liquidelectrolyte. The anode active material may be the same as or differentfrom the cathode active material. Since there are great amounts of pores(80-99%) relative to the pore walls (1-20%), very little space is wasted(“being wasted” means not being occupied by the electrode activematerial and electrolyte), resulting in high amounts of electrode activematerial-electrolyte zones (high active material loading mass).

Schematically shown in FIG. 1(B) is an embodiment of the presentlyinvented supercapacitor cell having large electrode thicknesses andlarge length; there is no limitation on the thickness or length of thecell. Both the electrically conductive porous layer in the anode (e.g.anode current collector foam) and the conductive porous layer at thecathode (e.g. cathode current collector foam) have been impregnated withtheir respective wet electrode active material mixtures. As an example,a pore 250, in an enlarged view, is filled with the wet anode activematerial mixture containing graphene sheets 252 (an example of anelectrode active material) and liquid electrolyte layers 254 that areclosely packed in an alternating manner. Such a tight packing enables usto achieve a high tap density (packing density) of the active materialthat otherwise cannot be achieved by any existing process.

In such a configuration, the charges (electrons) only have to travel ashort distance (half of the pore size, on average; e.g. a fewmicrometers) before they are collected by the current collector (porewalls). Additionally, in each wet electrode active material mixture,graphene sheets are dispersed in a liquid electrolyte (i.e. eachgraphene sheet is surrounded by liquid electrolyte). Upon impregnationinto the pores of the porous layers (as the anode or cathode currentcollector), the slurry (wet mixture) remains in a dispersion orsuspension state, in which individual graphene sheets remain surroundedby the liquid electrolyte, totally eliminating the possibility ofgraphene sheets being fully re-stacked that otherwise would result inthe specific surface area being significantly reduced. Thus, thepresently invented process produces a totally unexpected advantage overthe conventional supercapacitor cell production process.

In a preferred embodiment, the graphene electrode material is selectedfrom pristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, or a combination thereof. The starting graphitic material forproducing any one of the above graphene materials may be selected fromnatural graphite, artificial graphite, mesophase carbon, mesophasepitch, mesocarbon microbead, soft carbon, hard carbon, coke, carbonfiber, carbon nanofiber, carbon nanotube, or a combination thereof.

The present invention also provides a lithium-ion capacitor (LIC) or asodium-ion capacitor (NIC), wherein at least one of the two electrodesis produced by the presently invented process. More preferably, both theanode electrode and the cathode electrode for the presently invented LICor NIC are made by the presently invented process described above. Thisinventive process includes (A) Continuously feeding a secondelectrically conductive porous layer to an anode material impregnationzone, wherein the second conductive porous layer has two opposed poroussurfaces and contain interconnected electron-conducting pathways and,preferably, at least 70% by volume of pores; and (B) Impregnating a wetanode active material mixture into the second electrically conductiveporous layer from at least one of the two porous surfaces to form anelectrode. For instance, the wet anode active material mixture containsa liquid electrolyte and an anode active material preferably selectedfrom prelithiated or pre-sodiated versions of graphite particles, carbonparticles, Si nanoparticles, Sn nanoparticles, or any other commonlyused anode active materials for lithium-ion batteries or sodium-ionbatteries. These anode active materials can be made into a fine particleform and multiple particles, along with conductive additive particles,can be readily mixed with a liquid electrolyte to form a wet anodeactive material mixture (e.g. in a slurry form) for impregnation into aconductive porous layer.

In a lithium-ion capacitor (LIC), the anode active material may beselected from the group consisting of: (a) prelithiated particles ofnatural graphite, artificial graphite, mesocarbon microbeads (MCMB), andcarbon; (b) prelithiated particles or coating of Silicon (Si), germanium(Ge), tin (Sn), lead (Pb), antimony (Sb), bismuth (Bi), zinc (Zn),aluminum (Al), nickel (Ni), cobalt (Co), manganese (Mn), titanium (Ti),iron (Fe), and cadmium (Cd); (c) prelithiated alloys or intermetalliccompounds of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al, or Cd with other elements,wherein said alloys or compounds are stoichiometric ornon-stoichiometric; (d) prelithiated oxides, carbides, nitrides,sulfides, phosphides, selenides, and tellurides of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and their mixtures or composites;and (e) prelithiated graphene sheets; and combinations thereof.Prelithiation can be accomplished electrochemically by using a compactmass of graphene sheets as the working electrode and lithium metal asthe counter electrode. Prelithiation may also be accomplished by addinglithium powder or chips along with the anode active material (e.g. Siparticles) and conductive additive particles into a liquid electrolyte.

In a sodium-ion capacitor, the anode active material contains apre-sodiated version of petroleum coke, carbon black, amorphous carbon,activated carbon, hard carbon, soft carbon, templated carbon, hollowcarbon nanowires, hollow carbon sphere, or titanate, or a sodiumintercalation compound selected from NaTi₂(PO₄)₃, Na₂Ti₃O₇, Na₂C₈H₄O₄,Na₂TP, Na_(x)TiO₂ (x=0.2 to 1.0), Na₂C₈H₄O₄, carboxylate based material,C₈H₄Na₂O₄, C₈H₆O₄, C₈H₅NaO₄, C₈Na₂F₄O₄, C₁₀H₂Na₄O₈, C₁₄H₄O₆, C₁₄H₄Na₄O₈,or a combination thereof.

In a sodium-ion capacitor, the anode active material contains a sodiumintercalation compound selected from the following groups of materials:(a) Sodium-doped silicon (Si), germanium (Ge), tin (Sn), lead (Pb),antimony (Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti),cobalt (Co), nickel (Ni), manganese (Mn), cadmium (Cd), and mixturesthereof, (b) Sodium-containing alloys or intermetallic compounds of Si,Ge, Sn, Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; (c)Sodium-containing oxides, carbides, nitrides, sulfides, phosphides,selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof; (d) Sodiumsalts; and (e) Graphene sheets pre-loaded with sodium or potassium.Pre-sodiation can be accomplished electrochemically by using a compactmass of graphene sheets as the working electrode and sodium metal as thecounter electrode. Pre-sodiation may also be accomplished by addinglithium powder or chips along with the anode active material (e.g. Snparticles) and conductive additive particles into a liquid electrolyte.

Bulk natural graphite is a 3-D graphitic material with each graphiteparticle being composed of multiple grains (a grain being a graphitesingle crystal or crystallite) with grain boundaries (amorphous ordefect zones) demarcating neighboring graphite single crystals. Eachgrain is composed of multiple graphene planes that are oriented parallelto one another. A graphene plane in a graphite crystallite is composedof carbon atoms occupying a two-dimensional, hexagonal lattice. In agiven grain or single crystal, the graphene planes are stacked andbonded via van der Waal forces in the crystallographic c-direction(perpendicular to the graphene plane or basal plane). Although all thegraphene planes in one grain are parallel to one another, typically thegraphene planes in one grain and the graphene planes in an adjacentgrain are inclined at different orientations. In other words, theorientations of the various grains in a graphite particle typicallydiffer from one grain to another.

The constituent graphene planes of a graphite crystallite in a naturalor artificial graphite particle can be exfoliated and extracted orisolated to obtain individual graphene sheets of hexagonal carbon atoms,which are single-atom thick, provided the inter-planar van der Waalsforces can be overcome. An isolated, individual graphene plane of carbonatoms is commonly referred to as single-layer graphene. A stack ofmultiple graphene planes bonded through van der Waals forces in thethickness direction with an inter-graphene plane spacing ofapproximately 0.3354 nm is commonly referred to as a multi-layergraphene. A multi-layer graphene platelet has up to 300 layers ofgraphene planes (<100 nm in thickness), but more typically up to 30graphene planes (<10 nm in thickness), even more typically up to 20graphene planes (<7 nm in thickness), and most typically up to 10graphene planes (commonly referred to as few-layer graphene inscientific community). Single-layer graphene and multi-layer graphenesheets are collectively called “nano graphene platelets” (NGPs).Graphene sheets/platelets (collectively, NGPs) are a new class of carbonnanomaterial (a 2-D nanocarbon) that is distinct from the 0-D fullerene,the 1-D CNT or CNF, and the 3-D graphite. For the purpose of definingthe claims and as is commonly understood in the art, a graphene material(isolated graphene sheets) is not (and does not include) a carbonnanotube (CNT) or a carbon nanofiber (CNF).

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 4, 2003) (U.S. Patent Pub. No.2005/0271574); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process forProducing Nano-scaled Platelets and Nanocomposites,” U.S. patentapplication Ser. No. 11/509,424 (Aug. 25, 2006) (U.S. Pat. Pub. No.2008-0048152).

In one process, graphene materials are obtained by intercalating naturalgraphite particles with a strong acid and/or an oxidizing agent toobtain a graphite intercalation compound (GIC) or graphite oxide (GO),as illustrated in FIG. 4(A) and FIG. 4(B) (schematic drawings). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes in a GIC or GO serves to increase theinter-graphene spacing (d₀₀₂, as determined by X-ray diffraction),thereby significantly reducing the van der Waals forces that otherwisehold graphene planes together along the c-axis direction. The GIC or GOis most often produced by immersing natural graphite powder (100 in FIG.4(B)) in a mixture of sulfuric acid, nitric acid (an oxidizing agent),and another oxidizing agent (e.g. potassium permanganate or sodiumperchlorate). The resulting GIC (102) is actually some type of graphiteoxide (GO) particles if an oxidizing agent is present during theintercalation procedure. This GIC or GO is then repeatedly washed andrinsed in water to remove excess acids, resulting in a graphite oxidesuspension or dispersion, which contains discrete and visuallydiscernible graphite oxide particles dispersed in water. In order toproduce graphene materials, one can follow one of the two processingroutes after this rinsing step, briefly described below:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (104), which are each acollection of exfoliated, but largely un-separated graphite flakes thatremain interconnected.

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (106) that typically have athickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nanomaterial by definition).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,112), as disclosed in our U.S. application Ser. No. 10/858,814 (Jun. 3,2004) (U.S. Patent Pub. No. 2005/0271574). Single-layer graphene can beas thin as 0.34 nm, while multi-layer graphene can have a thickness upto 100 nm, but more typically less than 10 nm (commonly referred to asfew-layer graphene). Multiple graphene sheets or platelets may be madeinto a sheet of NGP paper using a paper-making process. This sheet ofNGP paper is an example of the porous graphene structure layer utilizedin the presently invented process.

Route 2 entails ultrasonicating the graphite oxide suspension (e.g.graphite oxide particles dispersed in water) for the purpose ofseparating/isolating individual graphene oxide sheets from graphiteoxide particles. This is based on the notion that the inter-grapheneplane separation has been increased from 0.3354 nm in natural graphiteto 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakeningthe van der Waals forces that hold neighboring planes together.Ultrasonic power can be sufficient to further separate graphene planesheets to form fully separated, isolated, or discrete graphene oxide(GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% by weightof oxygen.

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers) pristine graphene,graphene oxide, reduced graphene oxide (RGO), graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene,doped graphene (e.g. doped by B or N). Pristine graphene has essentially0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight.Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.Other than pristine graphene, all the graphene materials have 0.001%-50%by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.).These materials are herein referred to as non-pristine graphenematerials.

Pristine graphene, in smaller discrete graphene sheets (typically 0.3 μmto 10 μm), may be produced by direct ultrasonication (also known asliquid phase exfoliation or production) or supercritical fluidexfoliation of graphite particles. These processes are well-known in theart.

The graphene oxide (GO) may be obtained by immersing powders orfilaments of a starting graphitic material (e.g. natural graphitepowder) in an oxidizing liquid medium (e.g. a mixture of sulfuric acid,nitric acid, and potassium permanganate) in a reaction vessel at adesired temperature for a period of time (typically from 0.5 to 96hours, depending upon the nature of the starting material and the typeof oxidizing agent used). As previously described above, the resultinggraphite oxide particles may then be subjected to thermal exfoliation orultrasonic wave-induced exfoliation to produce isolated GO sheets. TheseGO sheets can then be converted into various graphene materials bysubstituting —OH groups with other chemical groups (e.g. —Br, NH₂,etc.).

Fluorinated graphene or graphene fluoride is herein used as an exampleof the halogenated graphene material group. There are two differentapproaches that have been followed to produce fluorinated graphene: (1)fluorination of pre-synthesized graphene: This approach entails treatinggraphene prepared by mechanical exfoliation or by CVD growth withfluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation ofmultilayered graphite fluorides: Both mechanical exfoliation and liquidphase exfoliation of graphite fluoride can be readily accomplished.

Interaction of F₂ with graphite at high temperature leads to covalentgraphite fluorides (CF)_(n) or (C₂F)_(n), while at low temperaturesgraphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In (CF)_(n)carbon atoms are sp3-hybridized and thus the fluorocarbon layers arecorrugated consisting of trans-linked cyclohexane chairs. In (C₂F)_(n)only half of the C atoms are fluorinated and every pair of the adjacentcarbon sheets are linked together by covalent C—C bonds. Systematicstudies on the fluorination reaction showed that the resulting F/C ratiois largely dependent on the fluorination temperature, the partialpressure of the fluorine in the fluorinating gas, and physicalcharacteristics of the graphite precursor, including the degree ofgraphitization, particle size, and specific surface area. In addition tofluorine (F₂), other fluorinating agents may be used, although most ofthe available literature involves fluorination with F₂ gas, sometimes inpresence of fluorides.

For exfoliating a layered precursor material to the state of individuallayers or few-layers, it is necessary to overcome the attractive forcesbetween adjacent layers and to further stabilize the layers. This may beachieved by either covalent modification of the graphene surface byfunctional groups or by non-covalent modification using specificsolvents, surfactants, polymers, or donor-acceptor aromatic molecules.The process of liquid phase exfoliation includes ultrasonic treatment ofa graphite fluoride in a liquid medium.

The nitrogenation of graphene can be conducted by exposing a graphenematerial, such as graphene oxide, to ammonia at high temperatures(200-400° C.). Nitrogenated graphene could also be formed at lowertemperatures by a hydrothermal method; e.g. by sealing GO and ammonia inan autoclave and then increased the temperature to 150-250° C. Othermethods to synthesize nitrogen doped graphene include nitrogen plasmatreatment on graphene, arc-discharge between graphite electrodes in thepresence of ammonia, ammonolysis of graphene oxide under CVD conditions,and hydrothermal treatment of graphene oxide and urea at differenttemperatures.

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 4(B), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 4(B),different crystallites in a graphite particle are typically oriented indifferent directions and, hence, a particular property of amulti-crystallite graphite particle is the directional average value ofall the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 4(B)) are intercalated in an acid solution to producegraphite intercalation compounds (GICs, 102). The GICs are washed,dried, and then exfoliated by exposure to a high temperature for a shortperiod of time. This causes the flakes to expand or exfoliate in thec-axis direction of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as graphite worms 104. These wormsof graphite flakes which have been greatly expanded can be formedwithout the use of a binder into cohesive or integrated sheets ofexpanded graphite, e.g. webs, papers, strips, tapes, foils, mats or thelike (typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

Acids, such as sulfuric acid, are not the only type of intercalatingagent (intercalant) that penetrate into spaces between graphene planesto obtain GICs. Many other types of intercalating agents, such as alkalimetals (Li, K, Na, Cs, and their alloys or eutectics), can be used tointercalate graphite to stage 1, stage 2, stage 3, etc. Stage n impliesone intercalant layer for every n graphene planes. For instance, astage-1 potassium-intercalated GIC means there is one layer of K forevery graphene plane; or, one can find one layer of K atoms insertedbetween two adjacent graphene planes in a G/K/G/K/G/KG . . . sequence,where G is a graphene plane and K is a potassium atom plane. A stage-2GIC will have a sequence of GG/K/GG/K/GG/K/GG . . . and a stage-3 GICwill have a sequence of GGG/K/GGG/K/GGG . . . , etc. These GICs can thenbe brought in contact with water or water-alcohol mixture to produceexfoliated graphite and/or separated/isolated graphene sheets.

Exfoliated graphite worms may be subjected to high-intensity mechanicalshearing/separation treatments using a high-intensity air jet mill,high-intensity ball mill, or ultrasonic device to produce separated nanographene platelets (NGPs) with all the graphene platelets thinner than100 nm, mostly thinner than 10 nm, and, in many cases, beingsingle-layer graphene (also illustrated as 112 in FIG. 4(B)). An NGP iscomposed of a graphene sheet or a plurality of graphene sheets with eachsheet being a two-dimensional, hexagonal structure of carbon atoms. Amass of multiple NGPs (including discrete sheets/platelets ofsingle-layer and/or few-layer graphene or graphene oxide may be madeinto a graphene film/paper (114 in FIG. 4(B)) using a film- orpaper-making process. Alternatively, with a low-intensity shearing,graphite worms tend to be separated into the so-called expanded graphiteflakes (108 in FIG. 4(B) having a thickness >100 nm. These flakes can beformed into graphite paper or mat 106 using a paper- or mat-makingprocess, with or without a resin binder.

Despite the fact that individual graphene sheets have an exceptionallyhigh specific surface area, graphene sheets have a great tendency tore-stack together or to overlap with one another, thereby dramaticallyreducing the specific capacitance due to the significantly reducedspecific surface area that is accessible by the electrolyte. Thistendency to re-stack is particularly acute during the supercapacitorcell electrode production process. In this process, graphene sheets,along with other conductive additive and resin binder (e.g. PVDF), aredispersed in a solvent (typically NMP) to form a slurry, which is thencoated on a surface of a solid current collector (e.g. Al foil). Thesolvent is then removed (vaporized) to form a dried layer of activematerial electrode, which is then fed through a pair of rollers in acompression machine to consolidate the electrode layer. These drying andcompressing procedures induce severe graphene re-stacking. In many ofthe scientific reports, even though the graphene sheets in an originalpowder form were found to exhibit an exceptionally high specific surfacearea, the resulting electrode only shows an unexpectedly lower specificcapacitance. Theoretically, the maximum specific capacitance of asingle-layer graphene-based supercapacitor is as high as 550 F/g (basedon an EDLC structure, no redox pair or pseudo-capacitance), butexperimentally achieved values have been in the range of mere 90-170F/g. This has been a long-standing problem in the art ofsupercapacitors.

The present invention provides a highly innovative and elegant processto overcome this graphene sheet re-stacking issue. This invented processcompletely eliminates the need to go through slurry coating, drying, andcompressing procedures. Instead of forming a slurry containing anenvironmentally undesirable solvent (i.e. NMP), the process entailsdispersing graphene sheets in a liquid electrolyte to form a slurry ofelectrode active material-liquid electrolyte mixture. This mixtureslurry is then injected into pores of a conductive foam-based currentcollector; no subsequent drying and compressing are required and no orlittle possibility of graphene sheets re-stacking together. Furthermore,graphene sheets are already pre-dispersed in a liquid electrolyte,implying that essentially all graphene surfaces are naturally accessibleto the electrolyte, leaving behind no “dry pockets”. This process alsoenables us to pack graphene sheets (with electrolyte in between) in ahighly compact manner, giving rise to an unexpectedly high electrodeactive material tap density.

The graphene sheets used in the aforementioned process may be subjectedto the following treatments, separately or in combination:

-   -   (a) Being chemically functionalized or doped with atomic, ionic,        or molecular species. Useful surface functional groups may        include quinone, hydroquinone, quaternized aromatic amines,        mercaptans, or disulfides. This class of functional groups can        impart pseudo-capacitance to graphene-based supercapacitors.    -   (b) coated or grafted with an intrinsically conductive polymer        (conducting polymers, such as polyacetylene, polypyrrole,        polyaniline, polythiophene, and their derivatives, are good        choices for use in the present invention); These treatments are        intended for further increasing the capacitance value through        pseudo-capacitance effects such as redox reactions.    -   (c) deposition with transition metal oxides or sulfides, such as        RuO₂, TiO₂, MnO₂, Cr₂O₃, and Co₂O₃, for the purpose of forming        redox pairs with graphene sheets, thereby imparting        pseudo-capacitance to the electrode; and    -   (d) subjected to an activation treatment (analogous to        activation of carbon black materials) to create additional        surfaces and possibly imparting functional chemical groups to        these surfaces. The activation treatment can be accomplished        through CO₂ physical activation, KOH chemical activation, or        exposure to nitric acid, fluorine, or ammonia plasma.

We have discovered that a wide variety of two-dimensional (2D) inorganicmaterials can be used in the presented invented supercapacitors preparedby the invented direct active material-electrolyte injection process.Layered materials represent a diverse source of 2D systems that canexhibit unexpected electronic properties and high specific surface areasthat are important for Supercapacitor applications. Although graphite isthe best known layered material, transition metal dichalcogenides(TMDs), transition metal oxides (TMOs), and a broad array of othercompounds, such as BN, Bi₂Te₃, and Bi₂Se₃, are also potential sources of2D materials.

Non-graphene 2D nanomaterials, single-layer or few-layer (up to 20layers), can be produced by several methods: mechanical cleavage, laserablation (e.g. using laser pulses to ablate TMDs down to a singlelayer), liquid phase exfoliation, and synthesis by thin film techniques,such as PVD (e.g. sputtering), evaporation, vapor phase epitaxy, liquidphase epitaxy, chemical vapor epitaxy, molecular beam epitaxy (MBE),atomic layer epitaxy (ALE), and their plasma-assisted versions.

We have surprisingly discovered that most of these inorganic materials,when in a 2D nanodisc, nanoplatelet, nanobelt, or nanoribbon form,exhibit remarkable EDLC values, even though these inorganic materialsare normally considered as electrically non-conducting and, hence, not acandidate supercapacitor electrode material. The supercapacitance valuesare exceptionally high when these 2D nanomaterials are used incombination with a small amount of nano graphene sheets (particularlysingle-layer graphene). The required single-layer graphene can be from0.1% to 50% by weight, more preferably from 0.5% to 25%, and mostpreferably from 1% to 15% by weight.

In the instant invention, there is no limitation on the type of liquidelectrolytes that can be used in the supercapacitor: aqueous, organic,gel, and ionic liquid. Typically, electrolytes for supercapacitorsconsist of solvent and dissolved chemicals (e.g. salts) that dissociateinto positive ions (cations) and negative ions (anions), making theelectrolyte electrically conductive. The more ions the electrolytecontains, the better its conductivity, which also influences thecapacitance. In supercapacitors, the electrolyte provides the moleculesfor the separating monolayer in the Helmholtz double-layer (electricdouble layer) and delivers the ions for pseudo-capacitance.

Water is a relatively good solvent for dissolving inorganic chemicals.When added together with acids such as sulfuric acid (H₂SO₄), alkalissuch as potassium hydroxide (KOH), or salts such as quaternaryphosphonium salts, sodium perchlorate (NaClO₄), lithium perchlorate(LiClO₄) or lithium hexafluoride arsenate (LiAsF₆), water offersrelatively high conductivity values. Aqueous electrolytes have adissociation voltage of 1.15 V per electrode and a relatively lowoperating temperature range. Water electrolyte-based supercapacitorsexhibit low energy density.

Alternatively, electrolytes may contain organic solvents, such asacetonitrile, propylene carbonate, tetrahydrofuran, diethyl carbonate,γ-butyrolactone, and solutes with quaternary ammonium salts or alkylammonium salts such as tetraethylammonium tetrafluoroborate (N(Et)₄BF₄)or triethyl (methyl) tetrafluoroborate (NMe(Et)₃BF₄). Organicelectrolytes are more expensive than aqueous electrolytes, but they havea higher dissociation voltage of typically 1.35 V per electrode (2.7 Vcapacitor voltage), and a higher temperature range. The lower electricalconductivity of organic solvents (10 to 60 mS/cm) leads to a lower powerdensity, but a higher energy density since the energy density isproportional to the square of the voltage.

The ionic liquid is composed of ions only. Ionic liquids are low meltingtemperature salts that are in a molten or liquid state when above adesired temperature. For instance, a salt is considered as an ionicliquid if its melting point is below 100° C. If the melting temperatureis equal to or lower than room temperature (25° C.), the salt isreferred to as a room temperature ionic liquid (RTIL). The IL salts arecharacterized by weak interactions, due to the combination of a largecation and a charge-delocalized anion. This results in a low tendency tocrystallize due to flexibility (anion) and asymmetry (cation).

A typical and well-known ionic liquid is formed by the combination of a1-ethyl-3-methylimidazolium (EMI) cation and anN,N-bis(trifluoromethane)sulphonamide (TFSI) anion. This combinationgives a fluid with an ionic conductivity comparable to many organicelectrolyte solutions and a low decomposition propensity and low vaporpressure up to ˜300-400° C. This implies a generally low volatility andnon-flammability and, hence, a much safer electrolyte for batteries.

Ionic liquids are basically composed of organic ions that come in anessentially unlimited number of structural variations owing to thepreparation ease of a large variety of their components. Thus, variouskinds of salts can be used to design the ionic liquid that has thedesired properties for a given application. These include, among others,imidazolium, pyrrolidinium and quaternary ammonium salts as cations andbis(trifluoromethanesulphonyl) imide, bis(fluorosulphonyl)imide, andhexafluorophosphate as anions. Based on their compositions, ionicliquids come in different classes that basically include aprotic, proticand zwitterionic types, each one suitable for a specific application.

Common cations of room temperature ionic liquids (RTILs) include, butnot limited to, tetraalkylammonium, di-, tri-, andtetra-alkylimidazolium, alkylpyridinium, dialkyl-pyrrolidinium,dialkylpiperidinium, tetraalkylphosphonium, and trialkylsulfonium.Common anions of RTILs include, but not limited to, BF₄ ⁻, B(CN)₄ ⁻,CH₃BF₃ ⁻, CH₂CHBF₃ ⁻, CF₃BF₃ ⁻, C₂F₅BF₃ ⁻, n-C₃F₇BF₃ ⁻, n-C₄F₉BF₃ ⁻, PF₆⁻, CF₃CO₂ ⁻, CF₃SO₃ ⁻, N(SO₂CF₃)₂ ⁻, N(COCF₃)(SO₂CF₃)⁻, N(SO₂F)₂ ⁻,N(CN)₂ ⁻, C(CN)₃ ⁻, SCN⁻, SeCN⁻, CuCl₂ ⁻, AlCl₄ ⁻, F(HF)_(2.3) ⁻, etc.Relatively speaking, the combination of imidazolium- or sulfonium-basedcations and complex halide anions such as AlCl₄ ⁻, BF₄ ⁻, CF₃CO₂ ⁻,CF₃SO₃ ⁻, NTf₂ ⁻, N(SO₂F)₂ ⁻, or F(HF)_(2.3) ⁻ results in RTILs withgood working conductivities.

RTILs can possess archetypical properties such as high intrinsic ionicconductivity, high thermal stability, low volatility, low (practicallyzero) vapor pressure, non-flammability, the ability to remain liquid ata wide range of temperatures above and below room temperature, highpolarity, high viscosity, and wide electrochemical windows. Theseproperties, except for the high viscosity, are desirable attributes whenit comes to using an RTIL as an electrolyte ingredient (a salt and/or asolvent) in a supercapacitor.

In order to make a pseudo-capacitor (a supercapacitor that works on thedevelopment of pseudo-capacitance through redox pair formation), theanode active material or cathode active material may be designed tocontain graphene sheets and a redox pair partner material selected froma metal oxide, a conducting polymer, an organic material, a non-graphenecarbon material, an inorganic material, or a combination thereof. Manyof the materials that can pair up with reduced graphene oxide sheets arewell-known in the art. In this study, we have come to realize thatgraphene halogenide (e.g. graphene fluoride), graphene hydrogenide, andnitrogenated graphene can work with a wide variety of partner materialsto form a redox pair for developing pseudo-capacitance.

For instance, the metal oxide or inorganic materials that serve in sucha role include RuO₂, IrO₂, NiO, MnO₂, VO₂, V₂O₅, V₃O₈, TiO₂, Cr₂O₃,Co₂O₃, Co₃O₄, PbO₂, Ag₂O, MoC_(x), Mo₂N, or a combination thereof. Ingeneral, the inorganic material may be selected from a metal carbide,metal nitride, metal boride, metal dichalcogenide, or a combinationthereof. Preferably, the desired metal oxide or inorganic material isselected from an oxide, dichalcogenide, trichalcogenide, sulfide,selenide, or telluride of niobium, zirconium, molybdenum, hafnium,tantalum, tungsten, titanium, vanadium, chromium, cobalt, manganese,iron, or nickel in a nanowire, nanodisc, nanoribbon, or nanoplateletform. These materials can be in the form of a simple mixture with sheetsof a graphene material, but preferably in a nanoparticle or nano coatingform that that is physically or chemically bonded to a surface of thegraphene sheets prior to being formed into a wet active material mixture(e.g. in a slurry form) and impregnated into the pores of the conductiveporous layers.

In what follows, we provide some examples of several different types ofgraphene materials, other types of electrode active materials (e.g.activated carbon and select inorganic materials), redox pair partnermaterials, and porous current collector materials (e.g. graphite foam,graphene foam, and metal foam) to illustrate the best mode of practicingthe instant invention. Theses illustrative examples and other portionsof instant specification and drawings, separately or in combinations,are more than adequate to enable a person of ordinary skill in the artto practice the instant invention. However, these examples should not beconstrued as limiting the scope of instant invention.

Example 1: Graphene from Carbon/Graphite Fibers

Continuous graphite fiber yarns were cut into segments of 5 mm long andthen ball-milled for 48 hours. Approximately 20 grams of these milledfibers were immersed in a mixture of 2 L of formic acid and 0.1 L ofhydrogen peroxide at 45° C. for 60 hours. Following the chemicaloxidation intercalation treatment, the resulting intercalated fiberswere washed with water and dried. The resulting product is a formicacid-intercalated graphite fiber material containing graphite oxidecrystallites.

Subsequently, approximately 1/2 of the intercalated or oxidized fibersample was transferred to a furnace pre-set at a temperature of 1,000°C. for 30 seconds. The compound was found to induce extremely rapid andhigh expansions of graphite crystallites. Approximately half of theas-exfoliated graphite fibers were subjected to de-oxygenation at 1,100°C. for 20 minutes in a nitrogen atmosphere to obtain reduced exfoliatedgraphite. A small amount each of the two materials was separately mixedwith an aqueous ethanol solution to form two separate suspensions, whichwere subjected to further separation of exfoliated flakes using anultrasonicator. Both graphite oxide platelets and reduced GO platelets(RGO) were found to be well-dispersed in the aqueous solution.

Two separate processes were conducted to prepare supercapacitor cellsfeaturing GO, reduced graphene oxide sheets (RGO), and RGO-carbonmixtures as electrode active materials. One process is conductedaccording to the presently invented direct injection of activematerial-electrolyte mixture slurry into pores of foamed currentcollectors. For comparison, the other process is the conventional onethat includes the steps of electrode coating on solid current collectorsand drying, lamination of coated current collectors and a separatordisposed between the two current collectors, encasing of the laminatedstructure, and injection of liquid electrolyte into the encased cell.

In one series of samples, activated carbon (AC) particles andmulti-walled carbon nanotubes (CNT) were separately added into the GOand RGO suspensions, respectively, with an AC-to-GO ratio of 5/95 andCNT-to-RGO ratio of 10/90. The resulting suspension was then impregnatedinto the pores of foamed current collectors (Ni foam) having a 95%porosity. Supercapacitor cells containing pure GO or NGP alone as theelectrode active material were also made.

In the present study, electrode active materials were also chosen basedon graphene sheets in combination with an inorganic material, whichincludes nanodiscs, nanoplatelets, or nanosheets of an inorganicmaterial selected from: (a) bismuth selenide, (b) transition metaldichalcogenide, (c) sulfide or selenide of zirconium, molybdenum,titanium, cobalt, manganese, iron, and nickel; and (d) boron nitride.

Example 2: Preparation of Graphene Oxide (GO) and Reduced Graphene Oxide(RGO) Nanosheets from Natural Graphite Powder

Natural graphite from Huadong Graphite Co. (Qingdao, China) was used asthe starting material. GO was obtained by following the well-knownmodified Hummers method, which involved two oxidation stages. In atypical procedure, the first oxidation was achieved in the followingconditions: 1100 mg of graphite was placed in a 1000 mL boiling flask.Then, 20 g of K₂S₂O₈, 20 g of P₂O₅, and 400 mL of a concentrated aqueoussolution of H₂SO₄ (96%) were added in the flask. The mixture was heatedunder reflux for 6 hours and then let without disturbing for 20 hours atroom temperature. Oxidized graphite was filtered and rinsed withabundant distilled water until neutral pH. A wet cake-like material wasrecovered at the end of this first oxidation.

For the second oxidation process, the previously collected wet cake wasplaced in a boiling flask that contains 69 mL of a concentrated aqueoussolution of H₂SO₄ (96%). The flask was kept in an ice bath as 9 g ofKMnO₄ was slowly added. Care was taken to avoid overheating. Theresulting mixture was stirred at 35° C. for 2 hours (the sample colorturning dark green), followed by the addition of 140 mL of water. After15 min, the reaction was halted by adding 420 mL of water and 15 mL ofan aqueous solution of 30 wt % H₂O₂. The color of the sample at thisstage turned bright yellow. To remove the metallic ions, the mixture wasfiltered and rinsed with a 1:10 HCl aqueous solution. The collectedmaterial was gently centrifuged at 2700 g and rinsed with deionizedwater. The final product was a wet cake that contained 1.4 wt % of GO,as estimated from dry extracts. Subsequently, liquid dispersions of GOplatelets were obtained by lightly sonicating wet-cake materials, whichwere diluted in deionized water.

Surfactant-stabilized RGO (RGO-BS) was obtained by diluting the wet-cakein an aqueous solution of surfactants instead of pure water. Acommercially available mixture of cholate sodium (50 wt. %) anddeoxycholate sodium (50 wt. %) salts provided by Sigma Aldrich was used.The surfactant weight fraction was 0.5 wt. %. This fraction was keptconstant for all samples. Sonication was performed using a BransonSonifier S-250A equipped with a 13 mm step disruptor horn and a 3 mmtapered micro-tip, operating at a 20 kHz frequency. For instance, 10 mLof aqueous solutions containing 0.1 wt. % of GO was sonicated for 10 minand subsequently centrifuged at 2700 g for 30 min to remove anynon-dissolved large particles, aggregates, and impurities. Chemicalreduction of as-obtained GO to yield RGO was conducted by following themethod, which involved placing 10 mL of a 0.1 wt. % GO aqueous solutionin a boiling flask of 50 mL. Then, 10 μL of a 35 wt. % aqueous solutionof N₂H₄(hydrazine) and 70 mL of a 28 wt. % of an aqueous solution ofNH₄OH (ammonia) were added to the mixture, which was stabilized bysurfactants. The solution was heated to 90° C. and refluxed for 1 h. ThepH value measured after the reaction was approximately 9. The color ofthe sample turned dark black during the reduction reaction.

After chemical reduction of GO to become RGO, the dried ROG powder wasdispersed in an organic electrolyte (acetonitrile+N(Et)₄BF₄) to form anorganic RGO slurry. GO powder was separately dispersed in sulfuricacid-based electrolyte to form an aqueous GO slurry. The two slurrieswere separately impregnated into two separate sets of conductive porouslayers (Ni foam) to produce two separate EDLC supercapacitor cells.

For comparison purposes, conventional slurry coating and dryingprocedures were conducted to produce conventional electrodes. Electrodesand a separator disposed between two electrodes were then assembled andencased in an Al-plastic laminated packaging envelop, followed by liquidelectrolyte injection to form a supercapacitor cell.

Example 3: Preparation of Single-Layer Graphene Sheets from MesocarbonMicrobeads (MCMBs)

Mesocarbon microbeads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. MCMB (10 grams) wereintercalated with an acid solution (sulfuric acid, nitric acid, andpotassium permanganate at a ratio of 4:1:0.05) for 48-96 hours. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The intercalated MCMBs were repeatedly washed in a 5%solution of HCl to remove most of the sulfate ions. The sample was thenwashed repeatedly with deionized water until the pH of the filtrate wasno less than 4.5. The slurry was then subjected ultrasonication for10-100 minutes to produce GO suspensions. TEM and atomic forcemicroscopic studies indicate that most of the GO sheets weresingle-layer graphene when the oxidation treatment exceeded 72 hours,and 2- or 3-layer graphene when the oxidation time was from 48 to 72hours.

The GO sheets contain oxygen proportion of approximately 35%-47% byweight for oxidation treatment times of 48-96 hours. GO sheets weresuspended in water and then dried to produce GP powder. The GO powderwas then thermally reduced at 200-700° C. for varying periods of time toproduce samples of reduced graphene oxide (RGO) powder having an oxygencontent of approximately from 1% to 20%. These RGOs were used directlyas an EDLC-type supercapacitor electrode material or join a partnermaterial (e.g. metal oxide, conducting polymer, etc., bonded to RGOsurface) to form a redox pair in a pseudo-capacitance basedsupercapacitor. Both the presently invented process and conventionalprocess were used to produce supercapacitor cells, which were compared

Example 4: Preparation of Pristine Graphene Sheets (0% Oxygen) as aSupercapacitor Electrode Active Material

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free, non-halogenated and halogen-free, etc.) can lead to agraphene foam having a higher thermal conductivity. Pristine graphenesheets were produced by using the direct ultrasonication or liquid-phaseproduction process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson S450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.Pristine graphene is essentially free from any non-carbon elements.Pristine graphene sheets were then incorporated in a supercapacitorusing both the presently invented procedure of slurry injection intofoam pores and conventional procedure of slurry coating, drying andlayer laminating. Both EDLC and pseudo-capacitance types (redox pairwith polyaniline or MnO₂) of supercapacitors were investigated.

Example 5: CVD Graphene Foam-Based Current Collectors on Ni FoamTemplates

The procedure for producing CVD graphene foam was adapted from thatdisclosed in open literature: Chen, Z. et al. “Three-dimensionalflexible and conductive interconnected graphene networks grown bychemical vapor deposition,” Nat. Mater. 10, 424-428 (2011). Nickel foam,a porous structure with an interconnected 3D scaffold of nickel waschosen as a template for the growth of graphene foam. Briefly, carbonwas introduced into a nickel foam by decomposing CH₄ at 1,000° C. underambient pressure, and graphene films were then deposited on the surfaceof the nickel foam. Due to the difference in the thermal expansioncoefficients between nickel and graphene, ripples and wrinkles wereformed on the graphene films. Four types of foams made in this examplewere used as a current collector in the presently inventedsupercapacitors: Ni foam, CVD graphene-coated Ni form, CVD graphene foam(Ni being etched away), and conductive polymer bonded CVD graphene foam.

In order to recover (separate) graphene foam, Ni frame was etched away.In the procedure proposed by Chen, et al., before etching away thenickel skeleton by a hot HCl (or FeCl₃) solution, a thin layer ofpoly(methyl methacrylate) (PMMA) was deposited on the surface of thegraphene films as a support to prevent the graphene network fromcollapsing during nickel etching. After the PMMA layer was carefullyremoved by hot acetone, a fragile graphene foam sample was obtained. Theuse of the PMMA support layer was considered critical to preparing afree-standing film of graphene foam. Instead, we used a conductingpolymer as a binder resin to hold graphene together while Ni was etchedaway. It may be noted that the CVD graphene foam used herein is intendedas a foamed current collector to accommodate the carbon particles orgraphene sheets injected along with a liquid electrolyte. But, we havesurprisingly found that such a graphene foam, with or without aconductive polymer, is itself a supercapacitor electrode material. Sucha combination enables a maximized amount of active materialsincorporated in a supercapacitor cell.

Example 6: Graphitic Foam-Based Current Collectors from Pitch-BasedCarbon Foams

Pitch powder, granules, or pellets are placed in a aluminum mold withthe desired final shape of the foam. Mitsubishi ARA-24 mesophase pitchwas utilized. The sample is evacuated to less than 1 torr and thenheated to a temperature approximately 300° C. At this point, the vacuumwas released to a nitrogen blanket and then a pressure of up to 1,000psi was applied. The temperature of the system was then raised to 800°C. This was performed at a rate of 2 degree C./min. The temperature washeld for at least 15 minutes to achieve a soak and then the furnacepower was turned off and cooled to room temperature at a rate ofapproximately 1.5 degree C./min with release of pressure at a rate ofapproximately 2 psi/min. Final foam temperatures were 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C.(carbonized) under a nitrogen blanket and then heat treated in separateruns in a graphite crucible to 2500° C. and 2800° C. (graphitized) inArgon.

Example 7: Preparation of Graphene Fluoride Sheets as a SupercapacitorActive Material

Several processes have been used by us to produce GF, but only oneprocess is herein described as an example. In a typical procedure,highly exfoliated graphite (HEG) was prepared from intercalated compoundC₂F.xClF₃. HEG was further fluorinated by vapors of chlorine trifluorideto yield fluorinated highly exfoliated graphite (FHEG). Pre-cooledTeflon reactor was filled with 20-30 mL of liquid pre-cooled ClF₃, thereactor was closed and cooled to liquid nitrogen temperature. Then, nomore than 1 g of HEG was put in a container with holes for ClF₃ gas toaccess and situated inside the reactor. In 7-10 days a gray-beigeproduct with approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixedwith 20-30 mL of an organic solvent (methanol and ethanol, separately)and subjected to an ultrasound treatment (280 W) for 30 min, leading tothe formation of homogeneous yellowish dispersions. Upon removal ofsolvent, the dispersion became a brownish powder.

Example 8: Preparation of Nitrogenated Graphene Sheets as aSupercapacitor Electrode Active Material

Graphene oxide (GO), synthesized in Example 2, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s. The product was washed several timeswith deionized water and vacuum dried. In this method graphene oxidegets simultaneously reduced and doped with nitrogen. The productsobtained with graphene: urea mass ratios of 1:0.5, 1:1 and 1:2 aredesignated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogencontents of these samples were 14.7, 18.2 and 17.5 wt % respectively asfound by elemental analysis. These nitrogenated graphene sheets remaindispersible in water. The resulting suspensions were then dried toobtain nitrogenated graphene powder. The powder was mixed in a liquidelectrolyte to form a slurry for impregnation into pores of conductiveporous layers.

Example 9: Preparation of MoS₂/RGO Hybrid Material as a SupercapacitorActive Material

A wide variety of inorganic materials were investigated in this example.For instance, an ultra-thin MoS₂/RGO hybrid was synthesized by aone-step solvothermal reaction of (NH₄)₂MoS₄ and hydrazine in an N,N-dimethylformamide (DMF) solution of oxidized graphene oxide (GO) at200° C. In a typical procedure, 22 mg of (NH₄)₂MoS₄ was added to 10 mgof GO dispersed in 10 ml of DMF. The mixture was sonicated at roomtemperature for approximately 10 min until a clear and homogeneoussolution was obtained. After that, 0.1 ml of N₂H₄*H₂O was added. Thereaction solution was further sonicated for 30 min before beingtransferred to a 40 mL Teflon-lined autoclave. The system was heated inan oven at 200° C. for 10 h. Product was collected by centrifugation at8000 rpm for 5 min, washed with DI water and recollected bycentrifugation. The washing step was repeated for at least 5 times toensure that most DMF was removed. Finally, product was dried and madeinto an electrode. On a separate basis, several different amounts (5% to45% by weight) of MoS₂ platelets were combined with activated carbonparticles form a composite electrode for making a supercapacitor.

Example 10: Preparation of Two-Dimensional (2D) Layered Bi₂Se₃Chalcogenide Nanoribbons

The preparation of (2D) layered Bi₂Se₃ chalcogenide nanoribbons iswell-known in the art. For instance, Bi₂Se₃ nanoribbons were grown usingthe vapor-liquid-solid (VLS) method. Nanoribbons herein produced are, onaverage, 30-55 nm thick with widths and lengths ranging from hundreds ofnanometers to several micrometers. Larger nanoribbons were subjected toball-milling for reducing the lateral dimensions (length and width) tobelow 200 nm. Nanoribbons prepared by these procedures (with or withoutthe presence of graphene sheets or exfoliated graphite flakes) were usedas a supercapacitor electrode active material.

Example 11: MXenes Powder+Chemically Activated RGO

Selected MXenes, were produced by partially etching out certain elementsfrom layered structures of metal carbides such as Ti₃AlC₂. For instance,an aqueous 1 M NH₄HF₂ was used at room temperature as the etchant forTi₃AlC₂. Typically, MXene surfaces are terminated by O, OH, and/or Fgroups, which is why they are usually referred to as M_(n+1)X_(n)T_(x),where M is an early transition metal, X is C and/or N, T representsterminating groups (O, OH, and/or F), n=1, 2, or 3, and x is the numberof terminating groups. The MXene materials investigated includeTi₂CT_(x), Nb₂CT_(x), V₂CT_(x), Ti₃CNT_(x), and Ta₄C₃T_(x). Typically,35-95% MXene and 5-65% graphene sheets were mixed in a liquidelectrolyte and impregnated into pores of conductive porous layers.

Example 12: Preparation of MnO₂-Graphene Redox Pairs

The MnO₂ powder was synthesized by two methods (each with or without thepresence of graphene sheets). In one method, a 0.1 mol/L KMnO₄ aqueoussolution was prepared by dissolving potassium permanganate in deionizedwater. Meanwhile 13.32 g surfactant of high purity sodiumbis(2-ethylhexyl) sulfosuccinate was added in 300 mL iso-octane (oil)and stirred well to get an optically transparent solution. Then, 32.4 mLof 0.1 mol/L KMnO₄ solution and selected amounts of GO solution wereadded in the solution, which was ultrasonicated for 30 min to prepare adark brown precipitate. The product was separated, washed several timeswith distilled water and ethanol, and dried at 80° C. for 12 h. Thesample is graphene-supported MnO₂ in a powder form, which was dispersedin a liquid electrolyte to form a slurry and impregnated into pores of afoamed current collector.

Example 13: Evaluation of Various Supercapacitor Cells

In most of the examples investigated, both the inventive supercapacitorcells and their conventional counterparts were fabricated and evaluated.The latter cells, for comparison purposes, were prepared by theconventional procedures of slurry coating of electrodes, drying ofelectrodes, assembling of anode layer, separator, and cathode layer,packaging of assembled laminate, and injection of liquid electrolyte. Ina conventional cell, an electrode (cathode or anode), is typicallycomposed of 85% an electrode active material (e.g. graphene, activatedcarbon, inorganic nanodiscs, etc.), 5% Super-P (acetylene black-basedconductive additive), and 10% PTFE, which were mixed and coated on Alfoil. The thickness of electrode is around 100 μm. For each sample, bothcoin-size and pouch cells were assembled in a glove box. The capacitywas measured with galvanostatic experiments using an Arbin SCTSelectrochemical testing instrument. Cyclic voltammetry (CV) andelectrochemical impedance spectroscopy (EIS) were conducted on anelectrochemical workstation (CHI 660 System, USA).

Galvanostatic charge/discharge tests were conducted on the samples toevaluate the electrochemical performance. For the galvanostatic tests,the specific capacity (q) is calculated as

q=I*t/m  (1)

where I is the constant current in mA, t is the time in hours, and m isthe cathode active material mass in grams. With voltage V, the specificenergy (E) is calculated as,

E=∫Vdq  (2)

The specific power (P) can be calculated as

P=(E/t)(W/kg)  (3)

where t is the total charge or discharge step time in hours.

The specific capacitance (C) of the cell is represented by the slope ateach point of the voltage vs. specific capacity plot,

C=dq/dV  (4)

For each sample, several current density (representing charge/dischargerates) were imposed to determine the electrochemical responses, allowingfor calculations of energy density and power density values required ofthe construction of a Ragone plot (power density vs. energy density).

Shown in FIG. 5 are Ragone plots (gravimetric and volumetric powerdensity vs. energy density) of two sets of symmetric supercapacitor(EDLC) cells containing reduced graphene oxide (RGO) sheets as theelectrode active material and EMIMBF4 ionic liquid as the electrolyte.One of the two series of supercapacitors was prepared according to anembodiment of instant invention and the other was by the conventionalslurry coating of electrodes. Several significant observations can bemade from these data:

-   -   (A) Both the gravimetric and volumetric energy densities and        power densities of the supercapacitor cells prepared by the        presently invented method (denoted as “inventive” in the figure        legend) are significantly higher than those of their        counterparts prepared via the conventional method (denoted as        “conventional”). The differences are highly dramatic and are        mainly due to the high active material mass loading (>20 mg/cm²)        associated with the presently invented cells, reduced proportion        of overhead (non-active) components relative to the active        material weight/volume, no need to have a binder resin, the        ability of the inventive method to more effectively pack        graphene sheets into pores of the foamed current collector.    -   (B) For the cells prepared by the conventional method, the        absolute magnitudes of the volumetric energy densities and        volumetric power densities are significantly lower than those of        their gravimetric energy densities and gravimetric power        densities, due to the very low tap density (packing density of        0.25 g/cm³) of RGO-based electrodes prepared by the conventional        slurry coating method.    -   (C) In contrast, for the cells prepared by the presently        invented method, the absolute magnitudes of the volumetric        energy densities (27.8 Wh/L) and volumetric power densities        (10,171 W/L) are higher than those of their gravimetric energy        densities and gravimetric power densities, due to the relatively        high tap density (packing density of 1.15 g/cm³) of RGO-based        electrodes prepared by the presently invented method.    -   (D) The volumetric energy densities and volumetric power        densities of corresponding supercapacitors prepared by the        conventional process are 3.1 Wh/L and 1,139 W/L, respectively,        which are one order of magnitude lower. These are dramatic and        unexpected.

FIG. 6 shows the Ragone plots (both gravimetric and volumetric powerdensity vs. energy density) of symmetric supercapacitor (EDLC) cellscontaining activated carbon (AC) particles as the electrode activematerial and organic liquid electrolyte. The experimental data wereobtained from the supercapacitors that were prepared by the presentlyinvented method and those by the conventional slurry coating ofelectrodes.

These data also indicate that both the gravimetric and volumetric energydensities and power densities of the supercapacitor cells prepared bythe presently invented method are significantly higher than those oftheir counterparts prepared via the conventional method. Again, thedifferences are huge and are mainly due to the high active material massloading (>15 mg/cm²) associated with the presently invented cells,reduced proportion of overhead (non-active) components relative to theactive material weight/volume, no need to have a binder resin, theability of the inventive method to more effectively pack graphene sheetsinto pores of the foamed current collectors. The highly porous activatedcarbon particles are not as amenable to more compact packing as graphenesheets. Consequently, for AC-based supercapacitors, the absolutemagnitudes of the volumetric energy densities and volumetric powerdensities are lower than those of corresponding gravimetric energydensities and gravimetric power densities. However, the presentlyinvented methods still surprisingly enables the AC particles to bepacked with a significantly higher tap density (0.75 g/cm³) than what isachieved with the conventional slurry coating process (0.55 g/cm³) inthe present study.

Shown in FIG. 7(A) are Ragone plots of lithium ion capacitor (LIC) cellscontaining pristine graphene sheets as the cathode active material,prelithiated graphite particles as the anode active material, andlithium salt (LiPF₆)—PC/DEC as organic liquid electrolyte. The data arefor both LICs prepared by the presently invented method and those by theconventional slurry coating of electrodes. These data indicate that boththe gravimetric and volumetric energy densities and power densities ofthe LIC cells prepared by the presently invented method aresignificantly higher than those of their counterparts prepared via theconventional method. Again, the differences are huge and are mainlyascribed to the high active material mass loading (>15 mg/cm² at theanode side and >25 mg/cm² at the cathode side) associated with thepresently invented cells, reduced proportion of overhead (non-active)components relative to the active material weight/volume, no need tohave a binder resin, the ability of the inventive method to moreeffectively pack graphene sheets into pores of the foamed currentcollectors.

For the LIC cells prepared by the conventional method, the absolutemagnitudes of the volumetric energy densities and volumetric powerdensities are significantly lower than those of their gravimetric energydensities and gravimetric power densities, due to the very low tapdensity (packing density of 0.25 g/cm³) of pristine graphene-basedcathodes prepared by the conventional slurry coating method. Incontrast, for the LIC cells prepared by the instant method, the absolutemagnitudes of the volumetric energy densities and volumetric powerdensities are higher than those of their gravimetric energy densitiesand gravimetric power densities, due to the relatively high tap densityof pristine graphene-based cathodes prepared by the presently inventedmethod.

Shown in FIG. 7(B) are Ragone plots of sodium-ion capacitor (NIC) cellscontaining pristine graphene sheets as the cathode active material,pre-sodiated graphite particles as the anode active material, and sodiumsalt (NaPF₆)—PC/DEC as organic liquid electrolyte. The data are for bothLICs prepared by the presently invented method and those by theconventional slurry coating of electrodes. These data indicate that boththe gravimetric and volumetric energy densities and power densities ofthe NIC cells prepared by the presently invented method aresignificantly higher than those of their counterparts prepared via theconventional method. Again, the differences are dramatic and are mainlydue to the high active material mass loading (>15 mg/cm² at the anodeside and >25 mg/cm² at the cathode side) associated with the presentlyinvented cells, reduced proportion of overhead (non-active) componentsrelative to the active material weight/volume, no need to have a binderresin, the ability of the inventive method to more effectively packgraphene sheets into pores of the foamed current collectors.

It is of significance to point out that reporting the energy and powerdensities per weight of active material alone on a Ragone plot, as didby many researchers, may not give a realistic picture of the performanceof the assembled supercapacitor cell. The weights of other devicecomponents also must be taken into account. These overhead components,including current collectors, electrolyte, separator, binder,connectors, and packaging, are non-active materials and do notcontribute to the charge storage amounts. They only add weights andvolumes to the device. Hence, it is desirable to reduce the relativeproportion of overhead component weights and increase the activematerial proportion. However, it has not been possible to achieve thisobjective using conventional supercapacitor production processes. Thepresent invention overcomes this long-standing, most serious problem inthe art of supercapacitors.

In a commercial supercapacitor having an electrode thickness of 150-200μm (75-100 μm on each side of an Al foil current collector), the weightof the active material (i.e. activated carbon) accounts for about25%-30% of the total mass of the packaged cell. Hence, a factor of 3 to4 is frequently used to extrapolate the energy or power densities of thedevice (cell) from the properties based on the active material weightalone. In most of the scientific papers, the properties reported aretypically based on the active material weight alone and the electrodesare typically very thin (<<100 μm, and mostly <<50 μm). The activematerial weight is typically from 5% to 10% of the total device weight,which implies that the actual cell (device) energy or power densitiesmay be obtained by dividing the corresponding active materialweight-based values by a factor of 10 to 20. After this factor is takeninto account, the properties reported in these papers do not really lookany better than those of commercial supercapacitors. Thus, one must bevery careful when it comes to read and interpret the performance data ofsupercapacitors reported in the scientific papers and patentapplications.

Example 14: Achievable Electrode Thickness and its Effect onElectrochemical Performance of Supercapacitor Cells

One might be tempted to think the electrode thickness of asupercapacitor is a design parameter that can be freely adjusted foroptimization of device performance; but, in reality, the supercapacitorthickness is manufacturing-limited and one cannot produce electrodes ofgood structural integrity that exceed certain thickness level. Ourstudies further indicate that this problem is even more severe withgraphene-based electrode. The instant invention solves this criticallyimportant issue associated with supercapacitors.

Shown in FIG. 8 are the cell-level gravimetric (Wh/kg) and volumetricenergy densities (Wh/L) plotted over the achievable electrode thicknessrange of the activated carbon-based symmetric EDLC supercapacitorsprepared via the conventional method and those by the presently inventedmethod. In this figure, the data points are labelled as the gravimetric(♦) and volumetric (▴) energy density of the conventionalsupercapacitors and the gravimetric (▪) and volumetric (X) energydensity of the presently invented supercapacitors. The activatedcarbon-based electrodes can be fabricated up to a thickness of 100-200μm using the conventional slurry coating process. However, in contrast,there is no theoretical limit on the electrode thickness that can beachieved with the presently invented method. Typically, the practicalelectrode thickness is from 10 μm to 5000 μm, more typically from 50 μmto 2,000 μm, further more typically from 100 μm to 1,000 μm, and mosttypically from 200 μm to 800 μm.

These data further confirm the surprising effectiveness of the presentlyinvented method in producing ultra-thick supercapacitor electrodes notpreviously achievable. These ultra-thick electrodes lead toexceptionally high active material mass loading, typicallysignificantly >10 mg/cm² (more typically >15 mg/cm², furthertypically >20 mg/cm², often >25 mg/cm², and even >30 mg/cm²). These highactive material mass loadings have not been possible to obtain withconventional supercapacitors made by the slurry coating processes.

Further significantly, the typical cell-level energy densities ofcommercial AC-based supercapacitors are from 3 to 8 Wh/kg and from 1 to4 Wh/L. In contrast, the presently invented method enablessupercapacitors containing the same type of electrode active material(AC) to deliver an energy density up to 11 Wh/kg or 8.2 Wh/L. Such anincrease in energy density has not been considered possible in thesupercapacitor industry.

Also highly significant and unexpected are the data summarized in FIG. 9for reduced graphene oxide-based EDLC supercapacitors. The cell-levelgravimetric and volumetric energy densities plotted over the achievableelectrode thickness range of the RGO-based EDLC supercapacitors (organicliquid electrolyte) prepared via the conventional method and those bythe presently invented method. In this figure, the gravimetric (♦) andvolumetric (▴) energy density of the conventional supercapacitors arebased on the highest achieved electrode tap density of approximately0.25 g/cm³, and the gravimetric (▪) and volumetric (X) energy density ofthe presently invented supercapacitors are from those having anelectrode tap density of approximately 0.75 g/cm³, by no means thehighest. No one else has previously reported such a high tap density forun-treated, non-activated RGO electrodes.

These data indicate that the highest gravimetric energy density achievedwith RGO-based EDLC supercapacitor cells produced by the conventionalslurry coating method is approximately 12 Wh/kg, but those prepared bythe presently invented method exhibit a gravimetric energy density ashigh as 31.6 Wh/kg at room temperature. This is an unprecedentedly highenergy density value for EDLC supercapacitors (based on the total cellweight, not the electrode weight or active material weight alone). Evenmore impressive is the observation that the volumetric energy density ofthe presently invented supercapacitor cell is as high as 23.7 Wh/L,which is more than 7 times greater than the 3.0 Wh/L achieved by theconventional counterparts.

Summarized in FIG. 10 are the data of the cell-level gravimetric andvolumetric energy densities plotted over the achievable electrodethickness range of the pristine graphene-based EDLC supercapacitors(organic liquid electrolyte) prepared via the conventional method andthose by the presently invented method. The legends include thegravimetric (♦) and volumetric (▴) energy density of the conventionalsupercapacitors (highest achieved electrode tap density of approximately0.25 g/cm³) and the gravimetric (▪) and volumetric (X) energy density ofthe presently invented supercapacitors (electrode tap density ofapproximately 1.15 g/cm³).

Quite significantly, these EDLC supercapacitors (without any redox orpseudo-capacitance) deliver a gravimetric energy density as high as 43.9Wh/kg, which already exceeds the energy densities (20-40 Wh/kg) ofadvanced lead-acid batteries. This is of high utility value since anEDLC supercapacitor can be charged and discharged for 250,000-500,000cycles, as opposed to the typical 100-400 cycles of lead-acid batteries.This achievement is very dramatic and totally unexpected in the art ofsupercapacitors. In addition, carbon- or graphene-based EDLCsupercapacitors can be re-charged in seconds, in contrast to thetypically hours of recharge time required of lead-acid batteries.Lead-acid batteries are notorious for their highly negativeenvironmental impact, yet the instant supercapacitors areenvironmentally benign.

Further significant examples include those data summarized in FIG. 11for the cell-level gravimetric and volumetric energy densities plottedover the achievable electrode thickness range of the pristinegraphene-based EDLC supercapacitors (ionic liquid electrolyte) preparedvia the conventional method and those via the presently invented method.The gravimetric (♦) and volumetric (▴) energy density are for thoseconventional supercapacitors (highest achieved electrode tap density ofapproximately 0.25 g/cm³) and the gravimetric (▪) and volumetric (X)energy density are for those inventive supercapacitors having anelectrode tap density of approximately 1.15 g/cm³. The presentlyinvented pristine graphene-based EDLC supercapacitors are capable ofstoring a cell-level energy density of 97.7 Wh/kg, which is 15 timesgreater than what could be achieved by conventional AC-based EDLCsupercapacitors. The volumetric energy density value of 112.3 Wh/L isalso unprecedented and is 30-fold greater than the 3-4 Wh/L ofcommercial AC-based supercapacitors.

Example 15: Achievable Active Material Weight Percentage in a Cell andits Effect on Electrochemical Performance of Supercapacitor Cells

Because the active material weight accounts for up to about 30% of thetotal mass of the packaged commercial supercapacitors, a factor of 30%must be used to extrapolate the energy or power densities of the devicefrom the performance data of the active material alone. Thus, the energydensity of 20 Wh/kg of activated carbon (i.e. based on the activematerial weight alone) will translate to about 6 Wh/kg of the packagedcell. However, this extrapolation is only valid for electrodes withthicknesses and densities similar to those of commercial electrodes (150μm or about 10 mg/cm² of the carbon electrode). An electrode of the sameactive material that is thinner or lighter will mean an even lowerenergy or power density based on the cell weight. Thus, it would bedesirable to produce a supercapacitor cell having a high active materialproportion. Unfortunately, it has not been previously possible toachieve an active material proportion greater than 30% by weight foractivated carbon-based supercapacitors or greater than 15% by weight forgraphene-based supercapacitors.

The presently invented method enables the supercapacitors to go wellbeyond these limits for all active materials investigated. As a matterof fact, the instant invention makes it possible to elevate the activematerial proportion above 90% if so desired; but typically from 15% to85%, more typically from 30% to 80%, even more typically from 40% to75%, and most typically from 50% to 70%.

As shown in FIG. 12, the cell-level gravimetric energy densities of theactivated carbon-based EDLC supercapacitors (with organic liquidelectrolyte) are plotted over the achievable active material proportion(active material weight/total cell weight), which are from 17.5% to75.4%. The instant invention also allows us to achieve a pristinegraphene content in a supercapacitor cell from 9.6% to 78.2% by weight,resulting in a gravimetric energy density from 9.6 Wh/kg to 97.7 Wh/kg.For instance, FIG. 13 shows the cell-level gravimetric energy densitiesplotted over the achievable active material proportion (active materialweight/total cell weight) in a supercapacitor cell for two series ofpristine graphene-based EDLC supercapacitors (all with organic liquidelectrolyte). FIG. 14 shows the cell-level volumetric energy densitiesplotted over the achievable active material proportion (active materialweight/total cell weight) for pristine graphene-based EDLCsupercapacitors (with ionic liquid electrolyte).

Example 16: The Electrochemical Performance of Supercapacitor CellsBased on Various Electrode Active Materials and/or Different Porous orFoamed Structures as Current Collectors

In order to evaluate the effect of the foam structure, we chose to useRGO as an example of electrode active material but vary the type andnature of the current collector. A wide variety of foams were selected,ranging from metal foam (e.g. Ni and Ti foam), metal web or screen (e.g.stainless steel web), perforated metal sheet-based 3-D structure, metalfiber mat (steel fibers), metal nanowire mat (Cu nanowires), conductivepolymer nanofiber mat (polyaniline), conductive polymer foam (e.g.PEDOT), conductive polymer-coated fiber foam (polypyrrole-coated nylonfibers), carbon foam, graphite foam, carbon aerogel, carbon xerogel,graphene foam (from Ni-supported CVD graphene), graphene oxide foam(obtained via freeze-drying GO-water solution), reduced graphene oxidefoam (RGO mixed with a polymer and then carbonized), carbon fiber foam,graphite fiber foam, and exfoliated graphite foam (exfoliated graphiteworms bonded by a carbonized resin). This extensive and in-depth studyleads to the following important observations:

-   -   (A) The electrical conductivity of the foam material is an        important parameter with a higher conductivity tending to result        in a higher power density and faster supercapacitor response        time.    -   (B) The porosity level is also an important parameter with a        higher pore content resulting in a larger amount of active        material given the same volume, leading to a higher energy        density. However, a higher porosity level can lead to slower        response time possibly due to a lower electron-conducting        capability.    -   (C) Graphite foams and graphene foams provide better response        time of a supercapacitor. However, metal foam enables more ready        formation of or connection to a tab (terminal lead). Two leads        are required in each cell.

A wide variety of electrode active materials for both EDLC and redoxsupercapacitors have been investigated, covering organic and inorganicmaterials, in combination with aqueous, organic, and ionic liquidelectrolytes. Summarized in the following table (Table 1) are someexamples of different classes of supercapacitors for illustrationpurposes. These should not to be construed as limiting the scope of theinstant application.

TABLE 1 Examples of supercapacitors prepared by the new method and theircounterparts prepared by the conventional slurry coating method.Electrode Active Gravimetric Volumetric thickness mass energy energyActive (μm) and loading density density Sample ID materials Electrolytemethod (g/cm2) (Wh/kg) (Wh/L) PPy-1 Polypyrrole- 2M NaCl in 535, 41 4624 cellulose H₂O new PPy-c Polypyrrole- 2M NaCl in 190, 13.2 8.8 3.1cellulose H₂O conventional RuO₂-AC-1 RuO₂ + AC 1M NaCl in 355, 16 37.726.6 H₂O new RuO₂-AC-c RuO₂ + AC 1M NaCl in 160, 7.2 11.6 7.7 H₂Oconventional NiO-RGO-1 NiO + 1M LiOH 555, 26.6 44.2 35.7 activated GO inH₂O new NiO-RGO-c NiO + 1M LiOH 160, 4.6 9.2 7.3 Activated GO in H₂Oconventional V₂O₅-NGn-1 V₂O₅ + THF + 627, 27.4 41.3 35.4 nitrogenatedN(Et)₄BF₄ new graphene V₂O₅-NGn-c V₂O₅ + THF + 175, 5.6 7.2 5.6nitrogenated N(Et)₄BF₄ conventional graphene MnO₂-RGO-1 MnO₂ + 1.0M 420,17.2 85 84 RGO Na₂SO₄ new MnO₂-RGO-c MnO₂ + 1.0M 187, 6.2 29 23 RGONa₂SO₄ conventional MoS₂-1 MoS₂/RGO Acetonitrile + 375, 25.8 42.3 35.8N(Et)₄BF₄ new MoS₂-c MoS₂/RGO Acetonitrile 155, 8.8 13.2 9.6 N(Et)₄BF₄conventional Ti₂CT_(x)-1 Ti₂C(OH)₂/ 1M LiOH 331, 15.6 15.8 12.7 quinoneGO in H₂O new Ti₂CT_(x)-c Ti₂C(OH)₂/ 1M LiOH 167, 4.5 6.7 4.2 quinone GOin H₂O conventional CNT-1 MWCNT EMI-TFSI 275  12.7 25.8 16.7 CNT-c MWCNTEMI-TFSI  95  2.3 6.2 3.2

These data further confirm the surprising superiority of the presentlyinvented method of producing supercapacitor cells in terms ofdramatically improving mass loading, electrode thickness, gravimetricenergy density, and volumetric energy density. The presently inventedsupercapacitors are consistently much better the conventionalsupercapacitors in electrochemical properties. The differences aresurprisingly dramatic.

In conclusion, we have successfully developed a new and novel class ofsupercapacitors that have unexpectedly thick electrodes (not previouslyachievable), large active material mass loading (not previouslyachievable), outstanding gravimetric energy density (not previouslyachievable), and unprecedentedly high volumetric energy density. Theinvented method of direct injection of an active material-electrolytemixture into foamed current collectors also overcomes the long-standingproblems associated with graphene sheet-based supercapacitors (i.e.inability to make thick electrodes, difficulty in preventing graphenesheet re-stacking, low tap density, and low volumetric energy density).

We claim:
 1. A supercapacitor cell comprising an anode, containing ananode active material, a cathode containing a cathode active material, aporous separator disposed between said anode and said cathode, and anelectrolyte in ionic contact with said anode active material and saidcathode active material, wherein at least one of said anode and cathodecomprise a foam current collector having interconnectedelectron-conducting pathways, said current collector having 70% to 99%by volume of pores, and wherein said pores contain said electrolyte andsaid anode active material or cathode active material residing therein.2. The supercapacitor of claim 1 wherein said foam current collector isselected from the group consisting of metal foam, metal web, metalscreen, perforated metal sheet-based three dimensional structure, metalfiber mat, metal nanowire mat, conductive polymer nanofiber mat,conductive polymer foam, conductive polymer-coated fiber foam, carbonfoam, graphite foam, carbon aerogel, carbon xerogel, graphene foam,graphene oxide foam, reduced graphene oxide foam, carbon fiber foam,graphite fiber foam, exfoliated graphite foam, and combinations thereof.3. The supercapacitor of claim 1 wherein said foam current collector hasa thickness from 100 μm to 5 mm.
 4. The supercapacitor of claim 1wherein said foam current collector comprises pores substantially havinga size from 10 nm to 50 μm.
 5. The supercapacitor of claim 1 whereinsaid anode or cathode comprises electrode active material with a loadinglevel from 7 mg/cm² to 30 mg/cm².
 6. The supercapacitor of claim 1,wherein said substantially all of said anode active material or saidcathode active material is less than 25 μm from a pore wall.
 7. Thesupercapacitor of claim 1, having a gravimetric energy density from 9.6Wh/kg to 97.7 Wh/kg.
 8. The supercapacitor of claim 1, having avolumetric energy density from 3 Wh/L to 12.3 Wh/L.
 9. Thesupercapacitor of claim 1, wherein said anode or said cathode containsgraphene sheets as the only electrode active material and does notcontain any other electrode active material.
 10. The supercapacitor ofclaim 1, wherein said anode or said cathode contains the followingmaterials as the only electrode active material in said anode orcathode: (a) graphene sheets alone; (b) graphene sheets mixed with acarbon material; (c) graphene sheets mixed with a partner material thatforms a redox pair with said graphene sheets to developpseudo-capacitance; or (d) graphene sheets and a carbon material mixedwith a partner material that forms a redox pair with said graphenesheets or said carbon material to develop pseudo-capacitance, andwherein there is no other electrode active material in said anode orcathode.
 11. The supercapacitor of claim 1 wherein said anode activematerial or cathode active material residing in the pores containssingle-layer graphene and few-layer graphene having from 1 to 10graphene planes; and said anode active material or cathode activematerial has a specific surface area from 500 m²/g to 2,675 m²/g whenmeasured in a dried state.
 12. The supercapacitor of claim 1 whereinsaid anode active material or cathode active material residing in thepores contains a graphene material selected from the group consisting ofpristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, boron-doped graphene,nitrogen-doped graphene, chemically functionalized graphene, physicallyor chemically activated or etched versions thereof, conductive polymercoated or grafted versions thereof, and combinations thereof.
 13. Thesupercapacitor of claim 12 wherein said chemically functionalizedgraphene contains surface functional groups selected from quinone,hydroquinone, quaternized aromatic amines, mercaptans, or disulfides.14. The supercapacitor of claim 1, wherein said anode active material orcathode active material residing in the pores contains a materialselected from graphene materials, activated carbon, activated mesocarbonmicro beads, activated graphite, activated or chemically etched carbonblack, activated hard carbon, activated soft carbon, carbon nanotube,carbon nanofiber, activated carbon fiber, activated graphite fiber,exfoliated graphite worms, activated graphite worms, activated expandedgraphite flakes, or a combination thereof, and said pores furthercomprise a redox pair partner material selected from a metal oxide, aconducting polymer, an organic material, a non-graphene carbon material,an inorganic material, or a combination thereof.
 15. The supercapacitorof claim 14, wherein said metal oxide is selected from RuO₂, IrO₂, NiO,MnO₂, VO₂, V₂O₅, V₃O₈, TiO₂, Cr₂O₃, Co₂O₃, Co₃O₄, PbO₂, Ag₂O, or acombination thereof.
 16. The supercapacitor of claim 14 wherein saidconductive polymer is selected from the group consisting ofpolyacetylene, polypyrrole, polyaniline, polythiophene, derivativesthereof, and combinations thereof.
 17. The supercapacitor of claim 14,wherein said inorganic material is selected from a metal carbide, metalnitride, metal boride, metal dichalcogenide, or a combination thereof.18. The supercapacitor of claim 14, wherein said metal oxide or saidinorganic material is selected from an oxide, dichalcogenide,trichalcogenide, sulfide, selenide, or telluride of niobium, zirconium,molybdenum, hafnium, tantalum, tungsten, titanium, vanadium, chromium,cobalt, manganese, iron, or nickel in a nanowire, nanodisc, nanoribbon,or nanoplatelet form.
 19. The supercapacitor of claim 14, wherein saidinorganic material is selected from nanodiscs, nanoplatelets,nano-coating, or nanosheets of an inorganic material selected from: (a)bismuth selenide or bismuth telluride, (b) transition metaldichalcogenide or trichalcogenide, (c) sulfide, selenide, or tellurideof a transition metal; (d) boron nitride, or (e) a combination thereof;wherein said nanodiscs, nanoplatelets, or nanosheets have a thicknessless than 100 nm.
 20. The supercapacitor of claim 14, wherein said anodeactive material or said cathode active material contains nanodiscs,nanoplatelets, nano-coating, or nanosheets of an inorganic materialselected from: (i) bismuth selenide or bismuth telluride, (ii)transition metal dichalcogenide or trichalcogenide, (iii) sulfide,selenide, or telluride of a transition metal; (iv) boron nitride, or (v)a combination thereof, wherein said nanodiscs, nanoplatelets,nano-coating, or nanosheets have a thickness less than 100 nm.
 21. Thesupercapacitor of claim 1, wherein said anode active material isdifferent from said cathode active material.
 22. The supercapacitor ofclaim 1, which is a lithium-ion capacitor or sodium-ion capacitor. 23.The supercapacitor of claim 22, wherein said lithium-ion capacitorcontains an anode active material selected from the group consisting of:a. prelithiated particles of natural graphite, artificial graphite,mesocarbon microbeads (MCMB), and carbon; b. prelithiated particles orcoating of silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony(Sb), bismuth (Bi), zinc (Zn), aluminum (Al), nickel (Ni), cobalt (Co),manganese (Mn), titanium (Ti), iron (Fe), and cadmium (Cd); c.prelithiated alloys or intermetallic compounds of Si, Ge, Sn, Pb, Sb,Bi, Zn, Al, or Cd with other elements, wherein said alloys or compoundsare stoichiometric or non-stoichiometric; d. prelithiated oxides,carbides, nitrides, sulfides, phosphides, selenides, and tellurides ofSi, Ge, Sn, Pb, Sb, Bi, Zn, Al, Fe, Ni, Co, Ti, Mn, or Cd, and theirmixtures or composites; e. prelithiated graphene sheets; f. andcombinations thereof.
 24. The supercapacitor of claim 22 wherein saidsodium-ion capacitor contains an anode active material selected from apre-sodiated version of petroleum coke, carbon black, amorphous carbon,activated carbon, hard carbon, soft carbon, templated carbon, hollowcarbon nanowires, hollow carbon sphere, or titanate, or a sodiumintercalation compound selected from NaTi₂(PO₄)₃, Na₂Ti₃O₇, Na₂C₈H₄O₄,Na₂TP, Na_(x)TiO₂ (x=0.2 to 1.0), Na₂C₈H₄O₄, carboxylate based material,C₈H₄Na₂O₄, C₈H₆O₄, C₈H₅NaO₄, C₈Na₂F₄O₄, C₁₀H₂Na₄O₈, C₁₄H₄O₆, C₁₄H₄Na₄O,and a combination thereof or said anode active material contains asodium intercalation compound selected from the group consisting of: a.sodium-doped silicon (Si), germanium (Ge), tin (Sn), lead (Pb), antimony(Sb), bismuth (Bi), zinc (Zn), aluminum (Al), titanium (Ti), cobalt(Co), nickel (Ni), manganese (Mn), cadmium (Cd), and mixtures thereof;b. sodium-containing alloys or intermetallic compounds of Si, Ge, Sn,Pb, Sb, Bi, Zn, Al, Ti, Co, Ni, Mn, Cd, and their mixtures; c.sodium-containing oxides, carbides, nitrides, sulfides, phosphides,selenides, tellurides, or antimonides of Si, Ge, Sn, Pb, Sb, Bi, Zn, Al,Fe, Ti, Co, Ni, Mn, Cd, and mixtures or composites thereof, d. sodiumsalts; e. graphene sheets pre-loaded with sodium or potassium; andcombinations thereof.